Very, very bad.
The camera is awkward and frustrating, the story line is a complete mess, and the whole thing takes about only four hours to complete. The graphics are so bad it looks like it should be a N64 game, not a next-gen title. There are only about five generic enemies through the whole game and only three bosses, and these are few and far between. It's total button mashing. Most of the game is spent just jumping from platform to platform, and occasionaly avoiding steam or fire.
There isn't any type of co-op! Totally lame.
The only reason I bothered playing past the third stage is for the achievement points, which are probably the easiest 1000 points you can get on the 360 right now.
Rent this game to get your achievement points, but DO NOT BUY TMNT!!! I don't understand how anyone can rate this above a 5.0!!!