Overall, I wish the movie was actually an entire season. I liked a lot of the ideas, but felt that not enough time was spent developing them. It's my criticism with most elements of the show lately.
Some of the plot details were pretty wonk. Greg really had to not-fuck his dying son. No matter how hard Sugar denies the nature of fusing, it's impossible for me not to see it that way.
The diamonds seemed to forget the entirety of their last major story arc: That they didn't do right by Pink and didn't really know how to love her in a way that was beneficial to her growth. Then, they want to adopt Spinel as a not-Pink replacement because they want to love her like they love Pink.
Steven's friends weren't exactly supportive. You'd think Lapis would swoop in to at least spot weak Steven while he tries to free-climb the injector. She is doing fuck all in that scene.
I was into the musical numbers., though The cast was picked to be well-suited for them and it shows. I've always enjoyed the songs in the show, and Spinel's 30s influenced style was incorporated very well. The Takarazuka Revue reference at the end came as an amusing surprise too.