It was pretty good. Certainly not my favorite iteration of the turtles, but I didn't go into it expecting it to be.
As others have said Splinter was very much so the standout of the film. Great version of the character. The turtles themselves were less offensive than I thought they were going to be, Donnie's voice is still pretty bad but oh well. The dynamic between them was great, but the individual characterization of them was lacking-- also not a fan of having Leonardo have a crush on April(hated it with Donatello in the 2012 series, hate it here, too.)
April was alright. I think she was surprisingly funny at times, but I still think her character works far better when she's older than the turtles and plays a more maternal role.
Baxter was wasted. Most of the "villains"(can't really call most of them that tbh) outside of Super Fly felt like afterthoughts. Like Seth Rogen just wanted to throw a bunch of nostalgic mutants on the posters but didn't think to give much substance to any of them. Mondo Gecko was cool but that's about it. Bebop, Rocksteady, Gender-flipped Leatherhead, etc. didn't do much of anything other than have a few lines.
The movie itself looked great, the style and animation itself was awesome BUT some of the character design choices themselves were really off-putting. Some of the ugliest designs I've seen in anything tbh lol. Which hey, I guess is intentional but eh. Soundtrack also could have been a lot better.
It'll be interesting to see where this universe goes. It's important to remember we'll most likely be seeing the tv series premiere before any sequel to the movie.