5.0: I will always play this card. Period.
4.5: I will almost always play this card, regardless of what else I get.
4.0: I will strongly consider playing this as the only card of its color.
3.5: I feel a strong pull into this card's color.
3.0: This card makes me want to play this color. (Given that I'm playing that color, I will play this card 100% of the time.)
2.5: Several cards of this power level start to pull me into this color. If playing that color, I essentially always play these. (Given that I'm playing that color, I will play this card 90% of the time.)
2.0: If I'm playing this color, I usually play these. (70%)
1.5: This card will make the cut into the main deck about half the times I play this color. (50%)
1.0: I feel bad when this card is in my main deck. (30%)
0.5: There are situations where I might sideboard this into my deck, but I'll never start it. (10%)
BLUE
Ætherize

Seem a little pricey for what it does, but bounce is stronger than most people give it credit. This card's most straightforward use is to buy defensive decks time. It can also save you from your opponent's combat tricks, either when you're attacking or when you block. Considering Gruul's entire mechanic is combat tricks, this should be more relevant than usual. It's also brutal against Simic, resetting their precious Evolve counters. Finally, most players have a habit of saving their instant speed removals for your attack step, so this could punish them for that.
2.0
Agoraphobia

It's certainly no Mark of Eviction, but for defensive decks it effectively stops the biggest threat on the board from doing combat damage, though it does nothing against active or triggered abilities. 2.0 in defensive decks, 1.0 elsewhere. Unless you're lucky enough to have a Simic Manipulator, then this card gets unfair.
2.0
Clinging Anemones

A wall that threatens to lock down the ground in a couple turns if you're lucky. When walls have enough power to kill most stuff that they block, they become quite good. Obviously it shines the most in defensive decks, but I could see this even showing up in tempo / racing blue decks that try to win with a handful of evasive beaters.
2.0
Cloudfin Raptor

Inconsistent as it may be, no one can deny how scary a turn 1 Raptor will be. By turn 3 that lil' 0/1 is swinging in like a Sunspire Griffin! Yes, it requires you to curve out, but you can raise the likelihood of that happening by building your deck appropriately. Also an excellent Cipher carrier, due to flying and all that jazz.
3.0
Diluvian Primordial

5/5 flying is always good, even at the steep price of 7cmc. It will probably also snag a sorcery or removal from your opponent's GY, so you should get a 2for1 of sorts. Not as good as the white or red primordials, but still pretty solid.
3.75
Enter the Infinite

Even if you managed to cast this, you'd probably just end up killing yourself.
0.0
Frilled Oculus

A 1/3 at 2cmc is a decent enough early blocker, with the pump spell keeping it relevant into the mid/lategame. Solid.
2.5
Gridlock

This is no Sleep, but all the same it can be that lategame blowout that lets you close out the game. Situationally good, I suppose.
1.5
Hands of Binding
I love everything about this card. I love the art, love the board impact, love the casting cost. If there was a single "underappreciated but awesome basically everywhere" card for blue, I would say this is the one.
Tapping down two creatures for a full turn is pretty decent for 2cmc. But the fact that you're tapping down potential blockers makes it even easier to keep getting in there for damage and looping the effect over and over. I love cipher cards like this that actively help you trigger its cipher ability more and more. Keeping a creature (2 on the first turn) locked down works for defensive decks trying to buy time, offensive decks trying to race, and cipher decks trying to get their creatures in there to trigger the cipher abilities. It's just... I just love it, okay? Feel free to disagree, but I think this is the overall best cipher card in the set.
3.25
Incursion Specialist

Decent turn 2 blocker that becomes an excellent cipher carrier, or at least in theory. I don't know how often you can be casting two spells a turn to keep triggering the ability. My guess is, "less often than you'd hope." It's funny that hitting with Cipher, your objective with this card, is actually the easiest way to trigger the Specialist's ability.. kind of backwards, huh?
Still, it has potential to be an undercosted unblockable dude that hits for 3. I don't know how consistent that is, so I'm going to err on the side of caution and say it's inconsistent and rate appropriately. If you've got the ideal deck for him he shoots up to 3.0.
2.0
Keymaster Rogue

Obviously plays nice with Cipher, Extort, and Evolve. Even better if you can bounce one of the handful of good ETB creatures in Dimir like Balustrade Spy.
2.5
Last Thoughts

Overpriced situational Divination that may, even more situationally, become a card draw engine. Not horrible, but hardly exciting.
2.0
Leyline Phantom

Big dork that, um, triggers Evolve over and over, I guess? I don't know, I'm not terribly excited by this. Yes, 5/5 is big, but recasting a 5cmc creature over and over is rough on the mana.
1.5
Metropolis Sprite

Welkin Tern is good, flexible blue cards like Frostburn Weird are good, this is good.
2.5
Mindeye Drake

Being defensive and milling is what Dimir Mill wants to do, and this drake does both admirably. Outside of that archetype, however, it's merely okay. 3.0 in Mill, 2.0 - 2.5 elsewhere.
3.0
Rapid Hybridization

1cmc unconditional removal is good, but not the mutant baggage that comes afterward. This card gets MUCH better if paired up with repeatable bounce spells so you can get rid of the downside. Otherwise it's okay, flexible, not terribly exciting.
2.0
Realmwright

Cheap manafixing attached to a 1/1, yay? No.
1.0
Sage's Row Denizen

The blue workhorse for the mill strategy. This is one of the better ones, likely the best mill card at common. 3.5 in mill, 1.5 elsewhere.
2.5
Sapphire Drake

4/4 flyer for 6cmc is pretty darn good by itself. Oh, you're playing heavy Simic? Well, now the majority of your other creatures are flying, too. Awesome.
3.0
Scatter Arc

In the creature-heavy limited format, this card is not one that I'd want to run maindeck. It can be a solid sideboard card against creature-light decks, however.
1.0
Simic Fluxmage

I love the utility on this card. You can disperse your +1/+1 counters around, making you less susceptible to removal. It's a repeatable combat trick, making it much harder for the opponent to attack or block. And if you play a creature just 1 p/t too high to trigger the Fluxmage's evolve, you can put a counter on a different creature so it does trigger. Not to mention with some specific creatures where +1/+1 counters have added utility, like Simic Manipulator, Gyre Sage, or Fathom Mage, you can help feed those creatures more counters for more craziness.
Slow, but very powerful.
3.0
Simic Manipulator

Slow, repeatable, permanent Control Magic on a stick. Um, yes, very much yes. Also works very well with Simic Fluxmage.
4.0
Skygames

If I'm very desperate for cipher triggers, I might run this. Hopefully I won't have that problem, however.
1.0
Spell Rupture

Not quite Mana Leak, but I can see it being just as good most of the time. Obviously it works better in creature-heavy decks like Simic. I can see this being powerful in limited.
2.5
Stolen Identity

You're getting a copy of the best dude on the table, probably two copies, with the hope of getting more.. Even if you're not copying a bomb each time, it's easy to see how you can run away with the game if this isn't answered in a turn or two.
4.5
Totally Lost

Please, Wizards, make Fblthp a planeswalker. Grant me this one wish.
Flavor aside, it's an okay card.
2.0
Voidwalk

Plays nice with Evolve and the handful of good ETB creatures out there. Works best if you're Orzhov splashing blue for this and running that one creature with the ETB that makes a 4/4 flying angel. Oh god, that combo will be the nuts. The. NUTS!
Very niche card, pointless in most decks (0.5), strong in a specific few (2.5).
1.5
Way of the Thief

Situationally good (you need a Gate), and in a format with tons of removal. It's certainly no Holy Mantle, but if you're ciphering and got 3+ gates, go for it.
1.5
Bane Alley Broker

A looter that can block reasonably well and can turn into actual card advantage later on. Very solid. Gorgeous art, too.
3.0
Call of the Nightwing

A cipher card that produces its own evasive cipher carrier, sweet! It's certainly no (Talrand's Evocation), but it can still be mighty powerful at gumming up the board and activating cipher over and over.
3.0
Consuming Aberration

So, you want to mill? Well, this is the card for you! This Horror mills about 2 cards each time you cast a spell and, assuming you're a mill deck, will be a nice big fatty to block your opponent's stuff, aka keep you alive so that mill actually works. A 4.5 in mill decks, probably a 3.5 elsewhere.
4.5
Dimir Charm

All its options are reasonably useful, but nothing stands out as very powerful.
2.5
Dinrova Horror

4/4 is pretty big in limited. The bounce effect is always tempo gain with the added bonus of ruining a fat Evolve creature's day. Since your bouncing a card first, the discard will ALWAYS be relevant, and sometimes it'll be a bonafide removal on a big stick if your opponent had no cards in hand. I dig it.
3.5
Duskmantle Guildmage

Are you milling? Then he's a great addition, probably around 3.0 - 3.5. Not milling? Then he's an unexciting bear, 1.5.
3.0
Duskmantle Seer

It's a 4/4 flyer for 4cmc. That's amazing and enough reason to run it. Now truth be told, if you're Dimir then you're probably going for control and playing higher cost cards, so the lifeloss might be sucky, but you should still be winning the race anyway because you have a 4/4 flyer. If you're going for the racing / tempo cipher route, then this is exactly what you want.
4.0
Lazav, Dimir Mastermind

3/3 hexproof for 4cmc is already playable. The fact that he can be even better than that is amazing in my books.
3.75
Mind Grind

This is the biggest one-shot mill spell around. Are you milling? Then this is your lategame finisher. Otherwise, ignore it.
3.5
Mortus Strider

A decent combo with Sage's Row Denizen, I suppose. Maybe a sideboard card against Boros aggro that runs too many 1 toughness creatures?
1.0
Paranoid Delusions

Okay, this is the main workhorse for mill. Assuming you've got an evasive beater, you're milling for 6 on the first cast + swing, for only 2cmc. Then another 3 each other hit. Very nice. If you're serious about mill, this will probably be in your deck.
3.5
Psychic Strike

Cancel is Cancel. Nothing exciting, but in dedicated mill it gets a little better.
2.0
Soul Ransom

If it's a huge bomb then you probably won't keep it for long, but it's still a 1for4, which is nuts. And hey, if it is a huge bomb that you need to deal with, drawing two cards will go a long way to find the answer you need.
4.0
Whispering Madness

It's a mill card, that's for sure. How good it will be at that is hard to say. I'm a little skeptical though.
1.5
Coerced Confession

Overpriced for what it does. I hope they remembered the safe word.
0.5
Deathcult Rogue

Better evasion than flying. Yep, it's a cipher carrier!
3.0
Nightveil Scepter

It's a Sunspire Griffin that mills and grants you card advantage, all rolled up in one. Honestly, I'd rate it highly if it was just a Sunspire Griffin, but the added bonuses are great. I can't think of many 3drops that are better than this.
4.0
Dimir Keyrune

One of the best keyrune, up there with Orzhov. It's an excellent (though mana-intensive) cipher carrier, and that's what really counts. It helps pay for extort and other heavy-costing controlly cards that Dimir is likely to play. Oh yes, and it's mana-fixing for tricolor decks and such.
2.25
Dimir Guildgate

Because people asked me to rate the guildgates and dual lands, here goes. Each one gets the same rating. They're useful in dual color decks but I don't rate them as very high picks. They become far more important in tricolor decks, however.
2.0
Watery Grave

Dual lands are similarly rated with Guildgates in limited. They're better if you have a good amount of early drops, but they're worse if you're running cards that benefit from Gates.
2.0
CLOSING THOUGHTS
Dimir is a confused guild. It wants to mill and it wants to attack with evasive creatures, and it sorta wants to do them both. In fact, some of the best mill cards actually require to hit your opponent first. But if you're going out of your way to hit your opponent.. why not just kill them with damage? Dropping an opponent's lifetotal is almost always easier than milling, after all.
Most guilds have a unified goal. Boros wants to just attack and kill the opponent with a battalion of doom. Orzhov wants to defend and go lategame to win with extort. The vast majority of cards in their limited pool is devoted to that goal. Since Dimir has two conflicting goals, however, their limited pool is split between both, meaning you simply have less cards available to do what you want to do. That's the true weakness of the guild.
There's still hope, however. Even though there's less cards to work with, that doesn't mean that Dimir's cards are bad. In fact, they attain their goal very well. Here's a rough outline on how to build your two archetypes:
Dimir Cipher:



Take these...



Add these...
BAM! There's your core. Then add general Dimir "goodstuff," aka removal, good creatures, etc, and you've got yourself a strong goal-oriented deck.
Dimir Mill:



Take these...



And these...
BAM! You're milling like a beast.
If Dimir gets the reputation of being the "worst" guild, that will actually give it more power in drafts. People will avoid the mill / cipher cards while you snap them up. Then it doesn't matter that you have less of a card pool to work with, you're being passed all the best stuff anyway!
So chin up, fellow Dimir lovers. It's not as bad as people are making it out the be. Especially not at Pre-Release -- you have 5 packs to work with, and if you're milling then I'd say the promo is actually the very best of the bunch. Personally, this is the guild that I'm going to be running. Good luck!