Because everyone else is doing it, here's my impression of Serenity.
I've never seen Firefly (something I plan to correct over the next week or so), so I went into the cinema with nothing but a hearty respect for Joss Whedon, and some rather high expectations fueled by a lot of talking up on the Internet.
I left... conflicted.
I really enjoyed Serenity, but I enjoyed it despite its many flaws. On one hand, the movie was too ambitious. It tried to fit a season of television exposition into a two-hour movie, on a third of budget of comparable sci-fi blockbusters, and came out feeling cramped and rushed. On the other hand, the ambition of the movie also made the whole bigger than the sum of its parts: if Whedon had settled for less scope, or been more timid with his budgetary constraints, he would have made a lesser movie.
The best example I can think of was there were too many characters. Eight is a great number for a TV show, where each week you can shift the focus to a different set of relationships. For a movie (unless you're planning a trilogy of three-hour epics), you can't fit that many people on screen for enough time to give them the development they deserve. No character had enough time to truly stand up, and some (such as Mal's love-interest) could easily have been cut entirely, if it weren't for the need to keep the fans happy.
On the other hand, even given their limited opportunities, most of the characters were invested with far more depth than most sci-fi movies afford their heroes. You still got a good sense of who they were and where they were going. You knew enough to care about them. Time was even spent out fleshing out the bad-guy, who could easily have just been a cardboard Terminator, but for whom you were forced to feel a twisted, sickening empathy.
And that, ultimately, is what's good about Serenity. When was the last time you read a review of a science fiction movie that talked at length about the characters? Despite all its baggage, it's a good movie - in turns tense, enjoyable, exciting and funny. And if everyone goes see it, Whedon will be given more money to make the next one with less baggage attached.
Three and a half stars out of five.