Ok, on to the movie proper. As RJ mentioned in his comments, Serenity is based on the short-lived TV series, Firefly- created by Buffy & Angel creator, Joss Whedon. I haven't seen the show, so I was going into a packed theater full of true believers. In my opinion, that is a difficult position for a film maker - how do you walk the line between keeping the true believers happy and bringing in new viewers without having them feel lost or lecturing them on what's gone before? A good example is the original X-Men -- they spent a lot of time trying to explain what was going on, which made the movie drag.
Fortunately, Joss Whedon proves that his reputation as a writing/directing wunderkind is entirely appropriate. Whedon was able to tell the story in a way that educated you about what was going on but was appropriate for the story. Further, it was clear that these characters had a past that I wasn't filled on, but it only served to give them depth, rather than leaving me feel lost or left out. Overall, Whedon was able to avoid the pitfalls of bringing something from the small screen to the big screen and managed to do it in an artful way.
Aside from those potential problems, Serenity was very much in-line with my previous Whedon encounters: the plot was very interesting and engaging, the script and dialogue were extremely well written, there was a healty injection of humor in the appropriate spots, everything was uniformly well acted, the special effects were good but weren't the focus of the movie. I didn't really see any particularly weak areas. Rather, all the pieces fit together into an excellent film. I would pay full price to watch this movie, I'll probably buy it when it comes out on DVD. I've put Firefly into my NetFlix queue.
Overall, if you have any affinity for a high quality Sci-Fi movie, I would be quite surprised if you didn't love this movie. I hope Serenity does well because Whedon pulled off quite a feat. I'm looking forward to seeing much more from him in the future.