Having never played any of the previous AvP games, I gotta say that I thought it was pretty fun.
Being a marine is sufficiently scary, while being a Predator is sufficiently badass, and being a xenomorph is...just plain fun.
My only issue is not being able to look down the sights as a marine. seriously, what kind of shitty marine runs around firing from the hip when you are at risk of being eviscerated? oh boo hoo a couple of acid-blooded aliens and some gnarly hunters are after you, big fuckin deal, that's no reason to suddenly forget all your fancy military training, you jackass.