Serenity is good. Very good. Very witty. Very dark. People die, crazy things happen and most of the characters get a fair amount of screentim. Didn't like the final dark secret that River uncovered, it ruined one of the best things in the Firefly mythology. They killed off my favourite character. Like a punk. But it still rocked.
So sad though - apparently the comic (which deals with the Blue Hands dudes from the series) was originally meant to be an animated prequel a la Animatrix or Clone Wars. That'd have been 2 new Firefly stories for the price of 1. Denied dude!