Its okay, on the hardest difficulty it is fun in coop, because you have no HUD and you can die fast.
The Versus-Mode can be also very fun, there Team-Deathmatch, Survive (you have to survive as mariens and attack as aliens) or a mode like Left 4 Dead, where you (marines) have to do special missions and reach the exit or some mission goal, aliens have to stop them.
There 3 default aliens classes, the default alien you may know from the movies, a spitter who can like the name say, spit and the jumper who jump like the hunter from left 4 dead.
At all this game is okay, not te best but it can be fun for a short time.
Well, for me it has a better Multiplayer then the latest Aliens vs. Predator game had.