This is a multiplayer game only and I have three words for it: addictive, addictive, addictive
It's extremely fun to play, it's got a diversity in classes which allows for different modes of play. The two opposing sides also have some differences in the way they feel and play.
The game is the struggle between Strogg and GDF (Global Defence Forces) so it's set in the universe created in Quake 2 and continued in Quake 4. The Strogg are ofcourse the evil aliens trying to invade the earth and GDF is there to stop them.
The game offers experience based play over campaigns which each have 3 different maps which mostly also offer three main objectives per map. The classes are soldiers (you're basic fighter), medics (reviving and keeping the team up), engineers (building defensive turrets and repairing) , fields ops (suppressing bombardements) and covert ops (hacking, sniping, camping and disguising). These five classes allow for a lot of variety in the way you can play the game and in addition to all of this there are still vehicles, both ground and air, which makes for enormous dynamic gameplay.
There's a demo out that you can try and this is the game that finally got me off playing Counter-Strike.