So, I have now extendedly played the demo with all 3 races, and the more I play, the more I actually like it (and because I always win).
The Marine feels at first a bit slow and clunky (increase mouse sensitivity in options to 100), but with his pulse rifle, he can waste even a Predator in no time. Sure he has no visual support, and a cloaked Pred can sometimes be really missed, but if you're not a total dumbass and have some basic shooter skills, I'd say the Marine is the most powerful dude in the game.
The Pred isn't really that menacing, unless he gets his shoulder cannon or disc, then he becomes almost unstoppable. There's definately some balancing needed there. In melee, he's just about the same strength as an Alien.
Playing as Alien is the most fun, even though the transitions from surface to surface it total garbage. Just use jump to get on the ceiling, it's faster. Alas, the demo map isn't really that great for using ceiling crawling. The best weapon is still speed. I love it when I rush behind some dude and use a power kill. It's very satisfying.
From a technical point of view, AvP is very solid. Great graphics, great sound, great performance. But the matchmaking stuff just doesn't work currently, and the demo doesn't have a dedicated server either, so if you want to play, just join http://steamcommunity.com/groups/AVPI.
Verdict: Solid fun, but nothing great.
If the singleplayer is good, I'll prolly end up buying the game.