While I wasn't a huge fan of Firefly when it was on TV, I became more of a fan while watching the DVD with all of the episodes in the correct order.
The Serenity movie is mostly quite strong and impressive. I think the space battle near the end is the best I've ever seen (I preferred it to Sith's space battles). I'm not sure if it completely works or not; while the acting is generally quite good, Sean Maher (Simon) is all over the place, and Nathan Fillion (Mal) had trouble in his early scenes. However, he does have some strong scenes, particularly with Ron Glass (Shepard).
I particularly adore the women - Zoe (the terrific Gina Torres), Inara (Morena Baccarin) and Kaylee (Jewel Staite) and they were generally well-used. I was worried by the trailer since they hardly appear in the trailer, but, luckily, they really appeared in the movie. I've also always enjoyed the wonderful "opposites attract" chemistry that Zoe and Wash (Alan Tudyk) have, and that's also well-played in the movie. Joss Whedon's TV casting translated very well to the big screen.
Often when a TV show becomes a movie, usually the plot revolves around some secret. Sometimes the "secret" revealed is kind of silly. The secret at the heart of this movie is a doozy and it really worked for me (though, since seeing the movie, I've been rewatching the series on DVD, and it's virtually mentioned in the series but kind of in passing).
The main weakness in this movie is some confusion around the end, and a stupidly choreographed fight scene at the climax. There's a scene that's supposed to be quite dramatic, but all I could think of was the Galaxy Quest "This episode is badly written!!" scene, so it was inadvertently funny. Additionally, there's kind of a deus ex machina moment nearthe end that's irritating.
However, I think these things could be fixed before the movie is formally released this fall. This movie has so much going for it - superb cast chemistry, a well-formed, "lived in" future, jaw-dropping but not overused special effects and mostly intelligent writing. I'll be curious to go back after its released and see how it was tweaked.
This isn't really a spoiler, but I know some people think that it is, so stop here if you're at all sensitive to such things.
While this isn't Blake Seven, some characters you care about do not come back; Whedon is to be congratulated for being willing to really kill off some of his children, without using a Trekian cheat.