Bought it last week and two days ago I finally mustered up the courage to watch it.
As others have said, it's way more violent than I thought it would be, making it clear from the get-go that these aren't the Nicktoon 2012 Turtles, although it also was very obvious that looks-wise they were based on them. (Stocky Raphael, Lanky Donatello) A shame though that they hadn't used more familiar voice actors but I guess that's just budget issues making decisions here.
"EEYAAARGH! THIS HURTS, IT HURTS WAY MORE THAN I EXPECTED!"
One thing I always loved about the 2012 series is that they didn't pussyfoot around with how severe a mutation must feel to the one who's mutating. Another good example of that is in "City at war" where Karai changed into her Snake form and is seen convulsing as if going through a grand mal seizure.
So, I was going "They better not pussyfoot around in this movie" and I was not disappointed when seeing the mutations and hearing the persons who mutated screaming and crumpling up in pain.
I was laughing out loud with the mutated Elephant but when the T-rex appeared I was going "They overstepped it here."
The highlight would be Shredder, I loved the brooding, scheming quality they gave him and the fight against Batman was awesome.