personally, a 7 out of 10. playing the game, I completely understand why it doesn't have co-op in it. because it's the first game that actually lets you move around like a turtle. you're running across walls, jumping up buildings and defying gravity all the time.
the combat is pretty simplistic. my only complaints so far is that combat is held to certain areas where you essentially have to clear the room of enemies before moving on, opposed to having enemies laid out through the level. also, the boss fights I've encountered so far have been fairly lackluster. nothing to write home about.
but I like running around. it's more platformer than brawler. which is a nice change of pace, and captured the essence of being a ninja turtle. unfortunately it didn't capture the feeling of fighting as a team.
but hopefully it will lay the ground work for a better game with co-op down the road. then again, as this game was critically canned by the media and fans, they would most likely go back to making generic brawlers to appease the fans again. which wouldn't be too bad as long as it was XBL co-op.
I bought this for $15, and am happy with it so far. I'm more than halfway through the game and having fun.