Hot damn is this movie better than the first one! I don't understand why it's getting poor critical reviews, excepting if critics hate cartoons for some reason. Because make no mistake, this is the cartoon, simply in live-action/CGI form, right down to the closing credits having the TMNT cartoon them play as the first song during them (a somewhat modern cover, granted, but close enough to the original as to make little difference).
Better still, the Turtles are definitely the focus this time around rather than April O'Neil, with a lot of time spent on developing them as brothers and as a team. O'Neil is decidedly a secondary character; she has her own subplot but it doesn't intrude on the Turtles, who retain more screen time than her. Casey Jones also has his own subplot involving Beebop and Rocksteady, but it is, again, a secondary thing to the Turtles themselves.
Speaking of, Beebop and Rocksteady were amazing. Every scene with them is pure gold, both pre- and post-mutation. Krang was great, too (though I preferred the spelling "Kraang", but the movie goes with the one-A variant), as was The Shredder and Baxter Stockman. Kirai is somewhat underutilized as a character, but in this movie she's pretty much here just to be The Shredder's lieutenant and doesn't really need much development for that role.
Basically, if you liked the cartoon, you'll like this movie and should go and see it, help the poor thing out - it definitely needs the help. Kind of like the movie version of Speed Racer, which is the anime in live-action form and makes no attempts to justify or apologize for anything from the anime, Teenage Mutant Ninja Turtles: Out of the Shadows is the cartoon, and everything that entails.