Summary: It's trash. Don't buy it. Due to the game's age, the multiplayer primarily consists of players who bought the DLC. They all run around in DLC mechs and kill people who are on foot. Don't bother buying it unless you also plan on getting the DLC for it. Also, this is primarily a multiplayer game although it does have a mediocre campaign mode you can play to kill 4-5 hours.
Summary: I really didn't enjoy this game. I bought it mainly for the Multi-player but gave the campaign a shot too. The Multi-player is weak. The guns are far too simple and lack any punch whatsoever. The campaign is quite good fun, but way too linear and voice acting is elementary. The building aspect - which all the magazines talk about as the best thing since slice bread - becomes very old very quickly. The Mulitplayer maps are either far too large or far too small.
Conclusion: On paper, this looks like a game with a lot of potential. But it's actually a bit tedious. The single player has a plot that is incomprehensible and seems like it was cobbled together by a 10-year old. You finish the game pretty quicky and wonder why you bothered. Popular opinion has it that the online multiplayer is where the fun lies, but after a few hours of trying this, I couldn't see what all the fuss was about.