This game is good not brilliant. Let down by some flaws.
The campaign's are way too short. Completed them all within a day and i don't play games like mad unless they are perfect (Assassin's Creed 2 and Modern Warfare 2 for example).
Also the controls can sometimes be fiddly. Which kind of let the Aliens campaign down for me as i found the Alien to be the best out of the three.
The online is also not brilliant but its not terrible. Not being able to mute little kids online (unless there is a way and i haven't found out yet) and if you play species team death match if your last to join the match you'll be lumbered with the marine. Also there aren't enough maps (more hopefully released in DLC)
Now the good :
The predators trophy kills are brilliant. Seriously gory but after a while you'll get used to them. Also as the predator being able to record someone's voice to distract them to where you want is brilliant.
Playing as the Alien is by far the best. The speed of it is amazing. The one thing that i found out about the Alien very quickly is that it gets ripped to shreds by bullets very quickly. But when you approach things stealthy. Taking out the lights and hiding in the darkness to perform a far less gruesome trophy kill.
Finally the Colonial Marines which i played as last. The Motion Sensor in the bottom right of the screen and the constant beeping when the enemy gets close i terrifying. Espicially as for the first 5 minutes you don't encounter any Aliens. Also the ammo is very scarce which means you're constantly trying to save ammo. Blocking also plays a huge part in the campaign which works brilliantly. Being able to block and then knock the Alien back so you have time to reload is good.
and Finally the Multiplayer. It's fun but dont expect Modern Warfare 2 standard. The game modes are fun with my favourite's being Predator Hunt, Survivor and Infestation.