So, I just got outta the theaters, so here's my opinion. (also, haven't seen Bob's opinion yet, just saying)
Pros:
I think they got the turtle's personalities down pretty well, besides Mikey being a major perv for some reason
Most of the fights without shakeycam looked kinda nice.
uhh.. that's about it.
Cons (watch out, this is gonna be the brunt)
A mad scientist, but not Baxter Stockman for some reason
They brought back Vernon, but not Irma.
Shredder looks like Lord Zedd's and a Predator's love child.
Shredder is literally coated in knifes, but would rather punch and kick the defenseless heroes
the foot clan are just thugs and gangsters with guns
Karai is completely useless
Splinter just found "How to be a ninja for Dummies" in the sewer, or something?
Having to forcibly connect April to the origin story
The turtles look like live action Shreks
Everything about Donnie's design, pretty much
Megan Fox didn't even wear a redhead wig.
Splinter grossed me out, and every other shot of him was a very extreme close up.
Are Pizzas that fucking big in New York? (more of a nitpick, and if true, I'm jealous)
How the hell do the turtles sliding down a hill on their shells consistently faster than the cars?
Ralph's character arc is literally the same every movie.
and finally No Casey Jones, Rocksteady, Bebob, Krang, Hun, Stockman, or Bishop
whew. that's about it. I'm sure there were others, but that's just off the top of my head.