The only "criticism" I'd make of the movie is that it was too short.
It was a great movie but time constraints meant that a lot had to be left out. Shepherd Book barely got any screen time and Inara's role was substantially cut.
I just wish we could have seen that story play out over a couple of seasons (which I'm sure was the original intention).
It's not just on the same level, it surpasses the show! Don't get me wrong, I love Firefly, but for a short 14 episode run it has a surprising amount of dull episodes.
Yes, the movie wraps up a lot of stuff from the series and brings it full circle. I actually think the movie is better than the series overall. Plus the production values are noticeably better than the series. The quality of the film stock, lighting, etc. make a big difference.
I remember the one episode where they get robbed and end up naked a lot. That makes the show worth it. The movie however was my introduction to the show so I always think of the movie first. And Inara.