This game was the perfect definition of mediocre IMO. It's not a horrible game, but it's not really that great either. You'll have a decent amount of fun playing it, for an hour or so, then it gets monotonous and repetitive. Having coop for the campaign helps a little, but the campaign itself is so bland that unfortunately even the coop can't save it in the end.
If you want to look at the worst shooters on the 360, then see games like Legendary, Quantum Theory, and ShellShock 2. Unreal Tournament 3 is infinitely better than these other games. But that still doesn't make it a great game unfortunately, just not the bottom of the barrel.