The thing with this game is, you've got to look at who it was intended for. Its a game made for the fans of the first two, and of course the fans of the movies, but mostly the fans of the first two. Which is both great, and disappointing.
Its great because of all the nostalgia that it is coated in. Thick, gooey amounts of nostalgia topped with a gawd-awful amount of fan service.
That being said, where it falls down is the... well, not the lack of originality and innovation, but the adherence to what made the first ones great. Everyone loves familiarity, but I know alot of die-hard fans who were let down because of its over-abundence. I for one wish they'd taken a few more risks with certain aspects.
Single player is enjoyable... sort of but nothing special. The marine campaign was the best, the alien campaign was the worst and the predator campaign was far to linear for my liking.
Multiplayer is where this game stands on. Its frantic, its gimmicky and it tows the line between good, balanced multiplayer action and just downright frustration (Melee is cheap, Predators are cheaper)
Here's hoping Colonial Marines will be better.