The game is pretty terrible, honestly. Most of the things I thought would make a 4 player TMNT game just weren't there. This is a generic, boring, and repetitive beat-em-up. I'll try to hit the few things that before purchasing made me think "this game is going to rock" and the revelations that led to my disappointment.
1) "The turtle power upgrade system will be really cool". Reality: It's not cool at all. You "level up" (although nowhere I could find is your actual level displayed) and get skill points you can distribute. But instead of getting a certain number of points for each turtle, you get a global number of points to distribute. If you want to switch between the different Turtles (especially in single player) you're going to fail pretty miserably. Also, most "skill tree" unlocks don't seems to do much if anything - there doesn't "feel" like a difference between being low level and high level.
2) "Everything I love about the shows and comic books is here." The developers really pushed this: this game was made for the 30-something Turtles fan. Except, in reality, it's not. Despite having completely different (and sometimes creepy) graphics, all of the characters are exactly the same as they are in the brand new Nickelodeon show. And as if to add insult to injury, they didn't even get the same voice actors from the new show. There isn't much here for the 30-something TMNT fan, especially if you haven't watched the new show.
3) "It will probably have an interesting/epic story". Nope. Seems like it was written for a 5 year old. Seriously, the new show has better plots than this game on a weekly basis. Nothing to see here. My playthrough did last about 4 hours (which is fairly high for a $15 game) but I'd say at least 2 hours of that was grinding away on and endless number of bad guys and/or replaying parts after dying due to their terrible checkpoint system.
4) "I heard about this game over a year ago: the developers have probably had plenty of time to flush out all of the bugs by now." Wrong again. In addition to the AI being (just, in general) terrible, I encountered TONS of bugs. Once while fighting Shredder the first time, one of my AI turtle teammates was standing on his head and they were both spinning in circles wildly without moving. During another boss battle with Kurai at the construction site, the boss got stuck in a part of the map I couldn't access yet and just kept throwing bombs at me, causing me to eventually have to reload from the previous checkpoint about 10 minutes earlier. Finally, the little "mini-game" where the turtles try to hack stuff: at first I thought it was really cool. Donatello hacks things easier than the others, so use him. Until the last level: where even with Donatello's ability, the hacking terminals are literally impossible. Meaning you have to exit out and re-enter the hacking attempt until you find one that can actually be solved. And I don't mean "impossible" like "Oh man, that's really hard, I'd never be able to figure that out!" I mean "impossible" like "there is literally no way this can be accomplished". I wish I had a screenshot handy because it's hard to explain. Anyway, it required a fair bit of loading/reloading on some of the hacking terminals.
Well that about sums it up. I can't really think of any redeeming qualities this game has. Besides maybe that a lot of the achievements were really easy to get. But even that becomes like the chore that drives you to play longer when you really just want to give up on it.
Sorry for the long review. If you're a huge TMNT fan and have watched the new cartoon (like me) I guess I could see picking this up for 75% off and being okay with it. Don't pay full price though, unless you want to be disappointed.