Never heard of Firefly? Get your hands on the DVD.
Saw some, weren't that impressed? Get your hands on the DVD.
Loved it? Well, you don't need convincing.
Fox did an incredible disservice to this show by showing the episodes out of order. They still made sense on a very low level, i.e., there weren't a lot of plot points that were part of the continuing arc that ended up being out of order in the order they were aired, but this show is so much better if you see it in the order intended. In the aired episodes, characters would sometimes do things that either seemed out of character or really weird, or sometimes even just boring, that make so much more sense on the DVDs, even though there's a grand total of one scene (and only about 3 lines of that scene) that's different between the DVD and the aired version.
That said, this show is the most innovative thing I've ever seen on TV, even despite its mistreatment. It has nine main characters, all of which are fairly well developed in the pilot episode, who then grow and change but remain true to their characters as originally conceived throughout the rest of the series. I could talk about the incredible attention to detail for the special effects, etc., but special effects are getting really good, so that doesn't really set this apart from a lot of movies out there. The concept seems odd at first, but is amazingly well done, with each world they go to having its own quirks, charms and dangers, but it always comes back to the characters and their relationships with each other.