Sometimes what you bring into a movie makes a big difference in how it will be viewed and ultimately enjoyed. If you're a history buff you may be irritated with period inaccuracies in a war movie. A fan of a certain comic might hate a feature version because it's not what they remembered or wanted. So going into TMNT it has a lot riding against it in terms of fans' expectations and how critics or even the general non-fan population view the Turtles either from the animated TV series or men-in-suits live-action films. It's tough to do anything right in some critics' minds when your franchise started with grown men playing teenage mutant ninja turtles.
I come to TMNT as a person who was a big fan of the TV series as a kid and vaguely remember liking the original live-action film when it first came out. Therefore, I have knowledge of the franchise and expectations of what would make a good or bad TURTLES movie. Thus my recommendation of the film comes from that point of view. If you were a fan of the series (but maybe not a hardcore one, I can't say how it stacks up to the comics) then you will not be disappointed by this film. All others should either go in with an open-mind or take their baggage and fly over to another theater that's not playing this movie. TMNT isn't a great film, but it brought back fond memories of the characters that I loved in my youth.