Well, the campaign seems to be purely about playing as the "colonial marines". IF there is some kind of competitive multiplayer, it might give the option to play Xenomorphs, but from what we currently know i doubt it.
Xenomorphs, sweet, awesome, yes! Hopefully Sega won't fail and let the marines be able to wrestle them, lol, that'd be like marines being able to wrestle a pred or brute from halo XD (If it already shows such in the youtube video, I am unable to watch it due to my main access to internet being my psp, which can't youtube)
http://www.gamefaqs.com/boards/528912-shifters/61948324 You probably won't (and maybe rather wouldn't) know what topic this links to.
Pretty much, if the xenos get close enough to jump on you, the only hope you have, it would seem, is your team shooting it off of you before it kills you. With there being different types of attacks that the xenos can do, they are not handcuffed to just mauling a person to death; the videos I have posted reveal that you will have attack choices you will slot and can even upgrade kind of like how the marines can upgrade their weapons.
There are also power-ups for the xenos according to one of the videos which is similar to the marines getting access to a new weapon. Recalling the descriptions, it is some kind of breeding chamber stuff, and they were kind of vague on it.
Playing as a xeno, it is a third person perspective, but their senses allow you a view that lets you get a glimpse through walls(hearing, smell, yada yada). They will have access to vents and cracks with wall climbing as an option.
In one video where they discussed how only Gearbox people played the xenos for the pvp, Gearbox players would yell fish-out-of-water whenever a marine was separated from the group which they would in turn swarm and kill the person with ease.
If you have the time and the eventual internet connection better than what you have, the videos are worth a look.