This game is a third person action RPG essentially, with a laundry list of flaws I'll get too shortly.
I have spent over 36 hours playing this game, and after playing everything including ONLINE CO OP (yes it works), I just can't reccommend this game to anyone for the full price.
TMNT has you controlling all 4 turtles at once through a Story Mode that consists of only 4 stages, as well as some other gamestyles such as Challenge Mode, Arcade Mode, and Survival Mode. The game has Leaderboards for some of the gamestyles.
I do consider this game an action RPG as it has an experience and ability point system that allows you to increase the stats of the turtles, allowing them to have a larger variety of moves they can do in battle.
You control one turtle while the other turtles are A.I. controlled, but you can switch to them at almost anytime.
The highlights of this game for me was the somewhat fluid battle system which I enjoyed learning and the turtles relationship both in and out of battle. The turtles combine their attacks and specials together during battle, and talk about all kinds of things during the stages.The controls and battle system take some getting used too of course, and i actually have little complaints about that part of the game. They did well with the controls.
The ability to switch to any character during the game was a cool idea, and it allows you to use some of the turtles out of battle skills such as Don's increased control device hacking ability.
The game does have a great look and unique feel to it, but here we run out of good things to say...
The game has horrible video settings to choose from. On my computer, it only offered me three choices on display resolution and little else useful. There were no texture settings or shadows or anything like that unless you go into the .ini file. I found the game laggy on any setting other than 640x480 but I was using a 1.87 GHZ Dual Core with a Radeon HD 6450 1024 MB.
Expect a lot of conversation between the Turtles at the worst possible times generally. You can turn voices off though in settings if you want.
Online co op is a big highlight here, however it does not work as well as you would hope. Steam invites do not work, but hosting a quick match seems to help. You can only play story mode though. The game seems to have in game voice chat which worked for me.
The big issue I have with this game is the price, the bugs, and the amount of gameplay you get here. With only four Story Mode stages I found that to be pathetic, especially compared to the other Turtles games from the past. The maps are a decent lengh, but you'll breeze through them.
There are some other game modes I mentioned, but they are played in the same stages as Story Mode with the same enemies. It's just doing the same thing in a slightly different way.
Finally, there are many bugs I've seen in this game, such as interacting with team attacks when your across the map and kicking at nothing in the air, or using special moves and finding your weapon following you around like a floating object. http://steamcommunity.com/sharedfiles/filedetails/?id=488745809
How about spawning on the opposite side of a locked room because you were too far away from the other turtles and having to switch to get to the other side? (Not a big deal but shouldn't happen).
The camera I found to be REALLY bad. You will constantly need to adjust it as it likes to move around and zoom in and out in the heat of battle causing you view to be obstructed by floating crossbeams above you.
The ending was terrible too. Just kill him already, it's been like 20+ years guy.
In conclusion, I would say ONLY CONSIDER buying this game if its on sale and your a big fan of the TMNT series and just can't find ANYTHING better to play.