I think this is probably the best representation of the TMNT on the big or small screens. It's certainly the closest to the comics.
A warrior from Central America gained immortality 3,000 years ago, and his generals were turned to stone. 13 monsters were released into the world and created carnage. Fast forward to now, and the portal from which the warrior gained his immortality is going to open again and he's gathering the monsters. Meanwhile the Turtles have grown up a little. Leo is in Central America to learn how to be a good leader, Mikey's got an entertainment business, Donatello works in IT support and Raph is, well, Raph. Meanwhile April and Casey have an antiques business and the Foot Clan, sans Shredder, is selling their services to the highest bidder.
The design on the film is great. The animation clearly takes a cue from Incredibles and this works for it as you don't spend your time worrying about how the CGI figures don't look like real people. It is stylised and fun. The story is rather interesting, mixing themes of brotherhood with the big plot about the monsters and the immortal. It's fun and dramatic, with a great fight between Raph and Leo.
It also seems to follow the previous films without being slavishly continuity bound. Shredder is dead, and rather than rehash another resurrection, they wisely leave him out to let the characters grow a little.
And the action! It simply couldn't be achieved by actors in spongy suits. It's as kinetic as anime and very well coreographed.
It's not Citizen Kane, but I doubt anyone who is reading these reviews is expecting that. But I think it represents a more mature approach to the franchise that takes it slightly higher than just another opportunity to sell toys.