Saw this last night a really enjoyed it, one thing though to those who have'nt watched it yet and think yeah boyeee, lets go watch Coporal Hicks kick some alien butt while yelling "Marines we are leaving !" Dont ! It's not that kind of flick, it's a movie set in the Alien universe and since there is no "official" story (I forget the proper word to describe history of fiction) to the alien universe as there is in Star Wars or Star Trek. The writers and directors really dont have to follow any strict guidelines, thats what I really liked about it.
It leaves it open to to interpretation to what said universe should be like sometimes you win audiences and critics alike and sometimes you lose some and win some, what matters is going with an open mind. Lets face it everyone is a critic and have to critique just like everyones got to fart now and then...