That's the best thing about pre-releases. Wizards tests limited so much that they really try to get everything fairly evened out. There's no telling what's going to rule, and what's going to be least played. When Return to Ravnica was released, everyone thought Rakdos was going to be awful, but they ended up being a very strong archetype, and most of the guilds were very well balanced (Izzet did suck tho). Each guild has it's own strengths and weaknesses, but i do not think Dimir will be overpowered. Gruul and Boros have possible (unlikely, but possible) turn 5 or so wins, a place in the game where Dimir and Orzhov will still be setting up. It might play way better than how i see it, but i think Simic (without a splash) is sadly the Izzet of this set.
boros and gruul are going to be scissors, simic is going to be rock, and orzhov is going to be paper; from what ive seen dimir is going to be that weird kid in the corner that no one wants to play with...
It's still too early to tell, man. We've only seen some "bombs" of the set. We still haven't seen the bulk of commons and uncommons of this set. Those are what sets the pace in limited, so it's too early to make assumptions at this point...
I think Simic will surprise players in how good it is. I do think Boros and Gruul will be good. For promos, I think Boros has the best.
I think people need to detach their feelings from RTR for limited. Highly aggressive builds sucked for that format. There was lifegain and populated tokens slowing everything down. Simic will do the best mimicking that behavior with all the "big butts" on their dudes and the evolve mechanic. I think 2/3s and 1/4s and all that just became important. Simic has to create a scenerio where they are throwing away dudes trying to get in that last damage. One thing is for sure, I don't see a lot of removal.
simic is basically playing the bant control game, counter using blue to make sure the field is clear, throw inexpensive creatures to attack and develop, and probably splash or red/black for a little more control or pump to finish the game
Boros is a mixture between RDW and izzet combo. play like rakdos, and finish the game with a teleportal or two. Probably splash green or black for some pumps/utilities in case of boardwipes and other fun stuff.
Dimir's general strategy is control and mill. Cipher makes it easier for them, and splashing extort spells will make sure they won't get killed too early and stealing some of the simic spells will give it some consistency.
Gruul is a more balanced izzet. burns, combat tricks and aggroish creatures. I think this can stand alone.
Orzhov has a good laid out strategy, but not enough control cards, might need to splash blue to help them stall the game.
5 color or 4 color sadly can't play anymore b/c no chromatic lantern =( but with realmwright it'll be ok to splash an off-colour bomb or two
I think Dimir is that strongest because you can play the promo you get and by far has the best promo fpr play in limited.
Sealed limited is not dictated by the dumbest bomb, but deck's overrall consistency. A consistent Selesneya deck can beat a bomby Azorius deck all day. So your assertion that the promo makes Dimir the best guild in sealed, despite it having subpar, overcosted cards is blatantly false.