played the first two missions of all 3 factions and i really like the game (hated the demo).
some impressions (360) version:
marine:
-great atmosphere, directly taken from aliens
-nice lighting system
-the unlimited flares really help, otherwise the game is very dark (with the flashlight)
-everything is taken from aliens: sounds, art design, atmo, alien behavior, some set pieces.
-marine campaign has the best visuals so far, mainly because of the lighting.
alien:
-very nice opening sequence
-gameplay is very stealthy (as expected)
-alien plays very fast, no framerate issues
-some of the stuff is directly taken from the first avp movie (alien queen design).
predator:
-predator feeling is perfect (music, weapons, hud, sound effects: everything is like in the movies)
-jumping around is fun, the jumping cursor is a little clunky
-fighting aliens is not fun. they are hard to see (no alien vision mode so far) and the melee system doesnt feel right. you get used to it but i hope it gets better later on.
-fighting marines is awesome. feels a little bit like arkham asylum.
-mostly melee combat, you have guns but the ammo is very limited (so far).
the game runs very smooth on 360 (no tearing, almost no slowdown).
not the best looking game ever but the atmosphere is fantastic.