I will start by answering the preeminent question everyone has: No; this isn't Bayformers. You can breathe a sigh of relief if you were expecting a disaster that's Revenge of the Fallen bad. The plot will confuse nobody (save for the very young, maybe), the action is fluid and visible, Megan Fox actually does a good job, and although not all jokes hit, you won't be driven crazy by their stupidity. If you're worried about the allegations of penis and fart jokes, rest assured there is just a penis joke and a fart joke that pass quickly, and best of all, there's no racism to be found in any characterizations.
Unfortunately, this new take on the iconic reptilian roughhousers is grounded to the point of being tedious in many places. The revised origin of the turtles; loosely based on the Fred Wolf cartoon's take but essentially its own thing (a theme prevalent through much of this version), eats up time as it's revealed by the film's pedestrian human cast. When I said Megan Fox does a good job as April Oneil, I mean that she adequately conveys when she's happy, sad, moved to tears by a sweet moment (and to be fair, you might be, too), and terrified, but she's still not playing a very interesting character. Also along are Will Arnett as Vernon Fenwick, who is much more likable than his Fred Wolf counterpart but not nearly as great as he was as Batman, Whoopi Goldberg as April's boss, Bernardette Thompson, who is sadly no more likable than her Fred Wolf counterpart (though for whatever reason, a different race and gender), and the Foot Clan, almost entirely reinvented as a gun-toting, western terrorist organization. Some may like this change of pace, as it's at least different from the Foot in every other Turtle movie, but when Karai, a character whose most faithful adaptation was in 16-bit fighting games, is reduced to pointing a pistol at people and driving a Humvee, the change truly rears its ugly head. I have no idea why she's in this movie.
Questionable new elements aside, things progress more-or-less the way you know if you're familiar with the brand; April runs afoul of the villains, the Turtles save her, she faints, they wake her up and introduce themselves and their rat sensei, Splinter (played by Monk himself, Tony Shaloub), a flashback montage is played, pizza is eaten, and then the fight is on to defeat the Foot Clan. It's at that point that finally, the movie comes into its own.
I'm not ashamed to say that Platinum Dunes' skepticism-prone take on the series does a few things better than the past movies. Aside from the original books, I think this is the most violent I've seen the Turtles and especially their foes get, and although it will be discomforting to some, it was long overdue. Heavy, brutal, head-bashing, bone-smashing impacts are inflicted by both sides, and nobody screws around. People hold up the original 1990 movie as the ideal TMNT film, and also as a rare adaptation that tried to reconcile the original Mirage series and the vastly altered Fred Wolf cartoon, but even it contained an extremely awkward moment where a scene of the Foot attacking April's apartment was interrupted by Michelangelo challenging a Foot ninja to a "chuck-off", and both sides just stopped and watched. As is well-known, the other live-action movies just got more inane from there, but no such illogical cheese is present here. Jokes get in where they fit in, sometimes as mid-action quips, and a fair amount are even pretty funny, but there's no pausing to fool around when lives are at stake. There's a pretty great truck chase segment; not something any other TMNT film has had, and Shredder is all new levels of menacing. He's barely a character, but his amped-up, "old-meets-new" arsenal of technologically charged blades provides some great new fights with the heroes. He's not taken down in any cheap way, either; the Turtles all bring their A-game against him, and even Fox's April gets some nifty licks in. It's cool, it's exciting, and it's funny.
But then it's over. Much as the film seems to go on too long during the boring exposition, just when it's gotten good, it flies by too fast. The pacing is awkward enough that I wonder if things were filmed first and then arranged in some arbitrary order, because it seems like it just coasts at first, and then fires off all of its exciting bits in rapid succession; like some constipated machine gun. The closest it came to enraging me was in the form of scenes that, in the trailers, made it look like we were going to get more adventures, instead being throwaway gags at the end--though in all honesty, they're more likely sequel hooks.
In summary, this new take on the Turtles isn't a total loss. It gets some things right about the brand (or as right as they can be; given how many different versions there are), there are some fun, if brief and objectively trivial, nods to old fans, and at least some of the new things it brings to the table have merit. Still, while there's only so much one can do retelling the origin story of the Turtles (this is the seventh time; count them), this film disappoints even those modest expectations, and ultimately the highest praise I can give it is that I was left wanting more instead of less. Hopefully, what we got at the end of the first film promises more character development, and more fun exploration of the Turtle mythos in the inevitable sequels.
Then again, we remember what happened last time.