Metacritic averages Serenity's reviews as 74 out of 100. "Generally favourable", apparently. The Onion AV Club, who I find tend to have similar tastes to mine, likes it:
Like all Whedon's work, Serenity is steeped in snappy, irreverent one-liners and complicated character dynamics
So that's all good then. Well, perhaps not. The obsessed over at Whedonesque have had a trainspotter-like feverish devotion to picking over every last Box Office report that goes up on the web. Apparently it was second last week and only made $10million, or there abouts. This week's BO has dropped off by about 50% which, again apparently, isn't bad for a Sci-fi film starring people most movie goers could be forgiven for never having heard of. The speculation is that if the movie makes enough money then a sequel could be in the works. Fine. That the movie exists in the first place is a minor miracle and that it's has been glowingly reviewed is a surprise. Can't that be enough for now? (For an example of using the movies money making ability to bash the movie's fans, all the while protesting that you don't hate them, check out this fun example).
By the way, you can watch the first few, nine?, minutes of Serenity over the Internets right here. Plus, just for fun, check out the Whedonesque page where they discuss whether Joss's show and Serenity in particular are naughty, "twixt my nethers" indeed!