It's not bad, but it's not great either. Playing the campaign is pretty fun, but there are issues when you look around with the mouse because the movement is a little choppy at times and becomes REALLY slow when you're steering a vehicle. Even at maximum mouse sensitivity, these issues don't go away. I had to increase the sensitivity through my mouse driver.
Multiplayer has the same issues, but also has the problem of lack of players. You can still find populated games, but not as many as Halo 1 or COD4, which can make it hard to earn some of the multiplayer achievements. Still, it's pretty fun.
If you want to try it out, go ahead as it's not as horrible as some of the other games out there. It's only $20 now, so I'd say it's worth the money.