I've watched the movie yesterday. Melissa and Daveed are the true stars. Halle does great too, but her caracter has less field to play along (the songs are the moments she shines!). The sisters are completely irrelevant and I'd say aome can't act at all. I didn't fully get Ursula's past and her current situation, but she is so cool and creepy so I loved her regardless.
I did not like the ending (I haven't seen the animated film yet so I don't know if there's any difference), although it made sense from Ariel's point of view. Jonah also did good (his song sucks and I think he can't sing, that's why it was heavily auto tuned), but the romance felt too rushed to justified everything she was giving up for.
Anyways, I feel 65% ~ 70% is great for the movie itself. Cinderella and Cruella were better live actions for me, although this one is totally worth the watch imo (especially for the musical numbers - POYW, UTS and PUS are incredible!!!).
Also, the theater I attended loved and applauded it at the ending. It didn't have many kids, mostly women and teenagers. I know these people were the most excited for the movie, probably, but if their reaction means anything this will please its target audience (like me).
I'll definitely watch it again, now in IMAX/3D.