Such an underrated game it's not even funny!
Single Player: The three campaigns are all based on the same time and events, but are all very different in their own unique way. For example the marine campaign is about the marine coming onto the planet to help out the people who live on the planet. The predator campaign however is about defending the buildings/tools which predators from generations back built/designed from the marines. I love every campaign because each has cool little additions/nods to the movies. For example in the first mission of the predator you are in a jungle fighting some aliens. When I looked up to see the aliens coming from the trees and such I saw a body which was dangling from a tree, skinned. This is a nod to the movie Predator (amazing movie btw) where the predator skins the humans.
Multiplayer: The multiplayer is (imo) just as fun as the campaigns. In the multiplayer there are several different modes like deathmatch and all that jazz. However, there are 3 modes which I will talk about which are awesome. The first mode is infestation, where everyone's a marine except one player who is an alien. The alien's job is to kill the marines. Each time a marine is killed by the alien they become an alien also. Eventually there's only one marine left and it gets very intense for them. There's an achievement for being the only marine left at the end (there's a time limit). This is probably the hardest achievement in the game. The second mode that should be mentioned is Predator Hunt, where every player is a marine, except one who is a predator, kind of like infestation except with predators instead. This differs from infestation however in that when the predator kills a human, they get points, but the marine stays as a marine. However when a marine kills the predator, then they become the predator. This game is fun because as the predator you can mess with the marines planting mines and using your shoulder launcher at them. Finally the third mode worth mentioning is survivor, which is based off of the games like Nazi Zombies and Horde Mode (GoW2). The task is simple: you are a marine (with up to 3 other people) and you must fend off the aliens as they come. It's epic and can get really intense when you're alone in a dark area and your motion sensor is beeping wildly.
All in all this game is awesome, and is way underrated. DO NOT BELIEVE ANY PROFESSIONAL GAME WEBSITE REVIEWS. If you like the Predator/Alien movies, then you will like this game. If not, rent it and see how it is. I love it because it includes so many nods to the movies, which is always fun to know what they are. I gave it an 8.5 because though the game is awesome it does have some annoying AI (not much, but some), and the controls are a bit confusing at first.