THE HARDEST GAME IN EXISTENCE!!!
Don't get me wrong the game is sweet as hell, it's just a tad on the difficult side. First, though, to clarify the difficulty let me explain the differences between the Alien, Predator, and the Marine.

The Alien
The Alien is fast, and powerful. Fox decided to give the Alien very weak armor and practically no health. Yet the character is still broken as hell. The reason he's broken is because his attacks can kill the Marine and the Predator with usually one hit. Also, even if you get shot, you can just eat the person you just killed and get all your health back!

The Alien also has two different views. One is the standard mode. In this mode you see light and dark the same as you would if you were a Marine. Except for one thing, if you’re the Alien you can see the pheromones the different races give off. Marines appear as blue, predator as green, and other aliens red. This is handy in dark rooms as you can see this halo even in the dark.

The Navigator view is useful for navigating (hence the name) dark corridors. Now you are probably asking “Why shouldn’t I use this view all the time?” Well because you cant see anything past like 10 game feet and those lovely pheromones don’t show up. One other thing about the navigation view is that everything is in negative. Can be a little disorienting at times.

The Predator
The Predator is an interesting character to play. If you like sniping you will love the Predator. Just like in the movie you can turn invisible, and use all those nifty different views (as in thermal) and weapons. One thing that surprised the hell out of me was how weak the Predator was. One on one with a marine the Predator would walk away but with little health. And an Alien against a Predator, one tail swipe and the Predator is smeared.

But one on one is not where the Predator excels. He is the ultimate sniper. With a zoom you can use at anytime and that great little disappearing trick he’s a machine on a perch. The best weapon for the Predator by far is the speargun. Now there is one thing that does suck with the speargun. When you shoot you reappear. They did this because if you did not reappear then the Predator would be broken as hell.

There are 3 different special viewing modes for the Predator. One is used to pick up heat. This is used to see Marines. The other one is electrical. This one is used to see Aliens. The last one is used to see Predators. This one actually picks up all mechanical equipment as well

Marine
The Marine. What can I say about the Marine? Ever play Quake2? Then you’ve played as the Marine. The Marine is the weakest character in the game. Period. He has some cool things like a motion detector, which doesn’t work very well. He also has night vision, which doesn’t work very well. The motion detector doesn’t really let you know where the motion is coming from. It tries very hard to. But just doesn’t cut it. The night vision is so grainy that to be totally honest doesn’t let you see dick. One other really disappointing thing with the Marine is that his best weapon is his base one. It’s a machinegun/grenade launcher mixture. Quite handy. The other ones, like the sadar, at first sound neat but just aren’t very effective.

Conclusion

As you can probably tell the Alien is my favorite to play. It has that "I am hunting your ass down" aura about it. And when I play Quake, that's usually how I play. The Marine requires for you to keep a distance between you and the enemy. I don't like that. The Predator can be used as a in your face play style, but is best used as a sniper. The single player as stated previously is hard as hell(especially if you are the marine). I mean, imagine you are this Marine with this machine gun being hunted down by tons of Aliens. It's a little intense. Yet in lieu of all these games coming out where the single player is largely ignored, it is rewarding. I highely recommend this game to anyone, especially if they are a fan of the movies, for it follows the nature of the characters quite closely.

P.S.-Don't let my opinion of the Marine's keep you from trying them out. They just really don't fit my play style.