I remember when I used to play this game online. I liked the game, the campaign had its moments, but the online was kind of a let down. The classes were unbalanced, Predators has the upper hand.
Predators: they have high tech weapons that can kill you in one hit. They have cloak and stealth attacks. They can jump far distances and they have all these visions they can turn on so it's easier to spot and enemy.
Alien: the second best class IMO. They are fast as hell, and if you kill them to close you take the enemies down with you. They have stealth attack and their tail does a lot of damage.
Soldier: the soldier is a poor class IMO. Their ammo is very limited and the weapons doens't deal much damage. I just felt like even though I fought in large numbers with others, it just seemed like we didn't stand a big chance.
I am sure Colonial Marines will be much better.