I am a leaf on the wind
Let me explain -- I'm not among Joss Whedon's legions of worshippers... I've never watched Buffy or Angel, though I did see all of the Firefly episodes, even before I knew there'd be a movie.
Firefly was a good show.
Serenity is a really great movie. Go and enjoy it, but you really ought to be familiar with the TV series first. The unique Space/Western setting might seem strange if you go in cold.
I know it will garner comparisons to Star Wars, but really, the more appropriate comparison will be to Star Trek. Whedon supposedly had Star Trek in mind when creating Firefly, aiming to create a deep space, ship-based series that was entirely unlike 'Trek'.
This film does satisfy a lot of cravings that the Trek films rarely did...