I have seen Serenity, and it is good. I mean really good. A couple of people, I won't say who in the non-spoiler portion of this review, seem a little out of character at first if you're familiar with Firefly, but that fairly quickly resolves itself, and everyone settles into the roles we know so well.
If you're not a Firefly fan, I'm really not sure how approachable you'll find this story. I daresay that you would find the "character immersion" in the early part of the film to be rather like learning to swim by being dumped into the Arctic Ocean. I have to wonder if you can generate enough sympathy for the characters to really respond to later events in the movie the way long-time fans will.
I think Joss Whedon is trying to say something about the nature of fanaticism in this film, but I don't want to go into details, yet.
In my opinion, though, this is definitely a film worth watching whether you're a Firefly fan or not. I was surprised that it was only on in one theater at the (Sh)Opry Mills multiplex, and with only four showings per day. If that's a typical schedule, then the fact that it made second place in theatre earnings against movies showing on multiple screens per theatre says a lot.
Spoilers will abound if you start reading the comments...