Firefly is absolutely amazing. I didn't care for it much while it was on tv. It's definately not a show that should be played out of order and in the first time slot of the evening. Luckily, my wife loved it enough to want the dvds, and after I saw them in order, I was blown away. My other favorite being Farscape, I'm lucky in that they're so very different I don't need to worry about being forced to choose
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On Serenity: I don't see it being huge enough to warrent a return to the small screen, but if they don't screw over the adverts and such for the movie, then I think it will do numbers profitable enough to warrent a sequel, if Wheadon and the cast want to return. I can't find the numbers for the series dvd sales myself, although I'm sure they're out there, but with a budget of only 35 million, a box office in mid spring of 18 or so million seems very attainable.