Just sunk about 3 hours in to the marine campaign and 1 hour in to the predator campaign and I can give this game thumbs up already. The control mechanics are good, nothing is broken about the gameplay and the actual gunplay is quite solid. I had lots of fun playing the 4 hours I played and I haven't even started the MP which is supposed to be the best part.
You will like this game if:
You like games like Doom 3, FEAR, Quake 4...*hint corridor shooter
You like very challenging enemies and always being on your toes. I am playing on normal and running in to all sorts of trouble...
You have the patience to learn some new control layouts (for predator and alien)
You are the fan of the franchises involved
You won't like it if:
You are a graphics whore. Game looks average... 5/10. Sounds great though.
You want to chill out and explore...sandbox elements. There is none. Its a push start, aim, fire, turn off console type of game
You want to play with your friends. Game is missing campaign coop, local coop for mp (xbl/PSN coop only), split screen.
If you read these points and think this may be a game for you, or you just want to play as a bad ass predator, don't hesitate to rent it or pick it up. Revewers seem to be hammering the game but we all know that the review system is faulty. Metacritic does not consider your gaming taste when giving grades to games. I enjoy this game much more than Bioshock 2 last week for example...personal taste.