This is saying quite a lot, because Serenity had to do a lot of things simultaneously:
1) Establish the characters and their relationships for people who'd never seen the series, and make those people care about them.
2) Satisfy the fans who'd seen the series -- which meant that it couldn't risk boring or alienating them while establishing (1).
3) Be big, in a thematic sense. You don't want people to trek out to a movie theatre and feel like they're watching a two-hour TV show when they get there.
4) Be self-contained: a story in its own right.
5) Be part of the overall story the show had begun.
6) And of course, have all the things any good film needs: a good plot, engaging characters, generate excitement, etc.
It did all of this, and did it well. I highly recommend it. You don't need to have seen the TV show to enjoy it -- nor will you be disappointed by it if you have. Like the TV show, it's best enjoyed as "science fantasy" rather than "science fiction". Though in some respects it's more plausible than much sf, and it strains credulity considerably less than, say, Star Trek. Still, it definitely requires suspension of disbelief.