Fun movie, but overall pretty underwhelming. Visuals and music is really good, Ado's singing really carries the movie. Everything else is just a mess though. The story is not very well written and there's a bunch of cameos from previous characters for no other reason than to have them there, they serve no purpose to the plot at all. It's fun sure, but doesn't make for a good movie. The episodes which give some "backstory" to Uta did little to nothing to impact my enjoyment of the movie at all so wouldn't worry about watching them before the movie.
I enjoyed the movie and would give it a 6/10 score on pure enjoyment alone, but unless you're a big One Piece fan I wouldn't go out of my way to watch it.