It's a must-have for any TMNT fan. The game itself is great and stays true to the series without any redesign besides the obvious visual changes made to the turtles (which honestly aren't that bad). Certain game mechanics like counter-attacking can be clunky, but is forgivable based on the plethora of old-school style combos (Think original Street Fighters and MK), comedic team attacks, and a rather in-depth upgrade system (Which is completely unexpected for a Turtles game). Aside from upgrading combos, attack abilities, and passive skills, you can also have Donatello construct gadget weapons for each of the turtles, adding another fun dimension to the game.
This game comes across to me as a mix of old-school fighters, arcade beat-em-ups, and modern batman-esque fighting mechanics and upgrade system.
I haven't personally tried the co-op, though I imagine it will still be fun as I've managed to overcome many of the game's shortcomings as a polished game.
I wish that the game had been completed a little better, but overall it is WAY more than I ever expected to see in a TMNT game and I hope that there are more like it in the future.