MTG Gatecrash Simic Strategies for a Prerelease event
In regards to GTC Simic in general, here's this humble magic players breakdown of the playables. The only cards included here are commons since you really can't ever rely on seeing even uncommons. There's about 80 uncommon cards a set, and you're going to see a total of 18. So maybe three or four of those will be simic, and maybe one of those will be playable. If they are great, but let's focus on the bread (no pun intended) and butter of your guild first. 

If a card doesn't appear in this list, it's because I personally didn't think it would be a good fit for the "grow 'em" strategy, which is undoubtedly what simic will be playing. That doesn't necessarily mean it's bad (although it probably means I think it's bad in general), and I welcome challenges to anything present in or absent from the list. I am comfortable making the list because it's 1) objective based on certain criteria, 2) limited (only 34 cards of the 259 are eligible for the list) and 3) comprehensive (it's not just a list but the reasons why are included so someone could apply the same sorts of logic to other cards).

Green playables:

Creatures: 


Crocanura - 3 toughness reach is decent - flying is huge in limited. Evolving will make it hit higher power quickly.
Crowned Ceratok - 4 power trample is great. Giving all your evolve trample is great too. Three toughness makes it weaker to red aggro cards like a 3/1 or 3/2, but it's still a great card.
Disciple of the Old Ways - He's a bear, which is usually playable anyway. His bonus is great if you're splashing red.
Greenside Watcher - If you have 3 or more gates, he's worth playing.
Ivy Lane Denizen - Every green guy gets a +1/+1 counter. Note that this +1/+1 counter happens in parallel with evolve, which is excellent!
Slaughterhorn - A 3/2 for 3 is probably playable anyway, but having bloodrush is simple excellent.
Spire Tracer - He's not a flier, but he acts like one. Green rarely gets fliers, so this is excellent news. Expect him to be targets of +1/+1 counters and bloodrushes.
Wasteland Viper - A 1/2 for 1 is rather lame considering last set gave us Dryad Militant (although that was uncommon). The bloodrush component of this is still excellent.


Spells:


Forced Adaptation - You risk being 2-for-1ed with this, but this will make your creature scale well with your turns. Combos well with Ceratok.
Pit Fight (R/G hybrid) - Cheap instant-speed removal at common. Between evolve, big green creatures, and Wasteland Viper, you've got a lot of viable fighters in your colors. 
Verdant Haven - Everyone loves acceleration, but green traditionally has the best.


Blue Playables:

Creatures:


Cloudfin Raptor - will probably swing for 1 on turn 2 as you'd expect with a typical flier.    
Keymaster Rogue - This guy is great for triggering those +1/+1 counters all over the place.
Metropolis Sprite - he has the +1/-1 ability that Frostborn Wierd had in RTR, and that proved to be very effective. Putting it on only one color isn't as good, and having only two toughness is also not as good, but if you have ways to get counters on blue creatures, this is a good place for it. Otherwise leave this out of the game.
Leyline Phantom - good for triggering +1/+1 abilites, although it's late in the game, simic should be able to drag the game out to make him worth it. He's kind of like an expensive keymaster rogue.


Spells:


Hands of Binding - probably the best blue spell at common, this is a must play if you have good fliers or other evasion.
Last Thoughts - this is pretty expensive if you don't get a lot of uses out of the cipher component, but if it does work out for you it's excellent. If your opponent has a lot of removal, sideboard this out.
Spell Rupture - If you have the big green fatties of old, this is effectively a Cancel for 1U isntead of 1UU. That's a great thing.
Way of the Thief - Maro promised gates would mean more in this expansion than in RTR. The only real use of gate in RTR before was on a fairly crappy defender in black that really didn't fit the profile. This on the otherhand is a pretty good fit. +2/+2 and unblockable is good, but doing it for 4 is pretty expensive. On the other hand, if the unblockable creature has a couple ciphers on him, well now you're going to win the game. Congrats.


Simic Playables:

Creatures:


Drakewing Krasis - he's a 3/1 flample - since he's still green he combos well with Ivy Lane Denizen.
Shambleshark - 2/1 flash evolve for 2 - requires exactly UG so could be tough to drop on turn 2. But with flash, you can leave the mana open and your opponent may think he's safer for a turn longer than he really is. Great potential here.


Spells:


Bioshift - if someone kills your creature with a bunch of +1/+1 counters (see Forced Adaptation) you can save them quickly.