Hooray, I got to go to a preview of Serenity on Monday.
It is very good.
It suffers a bit compared to the series in that the characters don't have the same depth (probably not aided by the need to introduce them all for people who haven't seen the series).
It is different from the series in that it is a lot more grim. I know that the series wasn't as dark as Whedon originally intended, but the film's Mal (for example) is a lot more callous than he was before. Almost scary. The plot also has a lot of stuff about the Reavers, which is all pretty unpleasant.
The film deals with a big chunk of River's story, and closes it off pretty well. There's more there, but I wouldn't be surprised if any sequels deal with other characters primarily.
If you liked the series, see this. If you like SF/action movies, see it as well.
Oh yeah, there's an absolutely great space battle. Fast, chaotic and awesome. Better than anything else I've seen recently (i.e. Star Wars episode 3).