I have to say I'm a little disappointed with the Alien portion of AvP.
In previous games that was always my favorite storyline. It was great fun to be playing as a nightmarish monster. Terrific to see civilians running in terror and even the marines warily shining their lights into dark corners.
Unfortunately, the alien storyline is the weakest part of this new AvP game.
To start with, you only get 5 missions. The other two species get 6 missions each. So right off the bat it feels like you're missing out on some gameplay. Especially when the ending cinematic for the alien could so easily have been turned into a final mission.
The next problem I had was with the control scheme. You have to jump to enter air ducts... Always. Even if it doesn't make sense. You can't just run up the wall and into them... You have to actually press you jump key to enter them. This seems un-necessary. And it frequently makes things awkward as you have to be at the right range and have the vent in the middle of your screen and whatnot. Previous games made it much easier to duck in and out of the vents.
Transitioning from one surface to another is also awkward. You've got a transition button you have to press... Which doesn't register right away... You have to hold it in for a moment. And leaving "auto transition" enabled doesn't work at all because you wind up sticking to crates and things like that.
Your vision is also a little weird... You can see the enemies through walls and floors - but only if you're close enough. It isn't the same kind of omniscience that you had in previous games. And it makes for some surprises that shouldn't happen. And it's hard to tell how well-lit an area is. The alien itself can see in the dark just fine... And lights are a little blinding... But it is hard to tell when you're actually hidden in shadows or not.
The storyline itself is decent...
You play as "6" - an alien picked out for Bishop's domestication project. You escape from your cell, free a number of other aliens, kill lots of people, and generally wreak havoc. The entire alien infestation of this planet can really be traced back to the escape of 6.
Generally speaking, your objectives make sense. You smash computers and electronics to shut off lights and defenses, or to open doors and let your fellow aliens in or out.
You've also got some secondary objectives like harvesting all the civilians in the colony, or destroying all the containers of royal jelly.
Harvesting the civilians is fun, especially when they scream in terror and run for their lives. Or some of them actually kill themselves to avoid being harvested - which is tricky.
Destroying the containers of royal jelly isn't nearly as much fun... Nor does it really make sense. Nobody ever explains why these containers exist or why you want to destroy them. And they're just hidden in random places. Places that don't make any sense at all. Places that no human could ever stash a jar of royal jelly.
You get to kill lots of marines. You've got the ability to sneak up on someone and kill them quietly... And you've also got some very bloody not-so-quiet kills...
You have to fight a few predators. Two youngbloods at once, and then an elite predator.
And then there's a level that really made no sense at all...
You have to get into a locked building. In order to do this, you need to find a vent that leads into the building. To get into the vent you need to open a stone grate. And in order to do that you need to locate three different pressure plates and trigger them in the right order.
Yup, that's right... An alien solving a switch puzzle.
Makes no sense, really ruins the flow of the game.
Eventually the matriarch gets killed, and you suffer some kind of psychic backlash from it. You curl up in a ball and Bishop's military androids capture you. You're packaged up for shipment off-world somewhere.
Then there's a final movie that shows the hangar inside a large ship... With a dropship sitting there... And a few corpses strewn about. Shows an air duct with alien resin around it. Shows the beginnings of an alien hive aboard the ship.
Makes it fairly obvious that you escaped captivity and started a new hive aboard this ship... But that's just shown in a movie. Why didn't they make that the final playable level? Would have been far more satisfying...