This mediocre computer-generated action flick is packed full of monsters and swordfights, but the story-telling is about as deft as a turtle turned on its back, struggling to get up but going nowhere fast. It's not a terrible movie, but it's not terribly much fun either, even taken on the level of its own modest ambitions. Those seeking the real-deal would be better advised to watch THE INCREDIBLES again.
The big problem is that TMNT writer-director Munroe makes the fatal mistake of taking the Teenage Mutant Ninja Turtles too seriously: he really seems to believe he is making a whiz-bang superhero thriller; he forgets that the Turtles are basically a joke and no amount of CG pyrotechnics will ever turn them into the Dark Knight. So we get a ponderous story about a 3000-year old conqueror trying to round up his old comrades (turned to stone - don’t' ask!) and some monsters he unleashed upon the world during his quest for global domination. This intersects with a rather tired tale of turtle sibling rivalry as Leonardo returns from a trip to South America where he was supposed to learn how to become a better leader for his brothers, Donatello, Michelangelo, and Raphael.
Neither story generates much interest; TMNT relies on action to hold audience attention. However, this is sadly lacking. The computer animation allows the turtles to do literally anything, but so what? The zooming, flashing, leaping, slicing, dicing is all so obviously artificial that it can never generate any genuine thrills; the CGI is simply too blunt, too unimaginative, lacking the visceral impact of the best Japanese anime. (For a comparison point, check out the brief early fight in GHOST IN THE SHELL 2, which with a few deft moves puts everything in this film to shame.)
On top of everything else, the turtle design is awful - like a CG rendition of a steroid-abusing muscleman, their green skin makes them resemble the Incredible Hulk, with turtle shells strapped on like armor. These Teenage Mutant Ninja Turtles are definitely more Mutant than Turtle. The human characters are no better, particularly the women, whose ridiculously slim waistlines make them resemble literal Wasp-Women.
The screenplay gets in an occasional good line (as when April O'Neil claims her client has too much time on his hands - unaware that the man is literally immortal). But in the end, this computer-generated film lacks the tongue-in-cheek amusement of the old live-action films, in which half the fun was seeing how well the Jim Henson Muppet Factory could bring the comic book characters to life. TMNT is slick, perhaps even technically impressive, but it's a mechanical wind-up turtle that runs through the motions without the life of the real thing.