Serenity is based on Firefly, a show that did poorly on TV mainly because the US TV execs messed it about a lot, different times each week - even the dedicated had a hard time finding it. It looked like Firefly was dead, but strong DVD sales due to word of mouth recommendations gave the execs pause for thought, and in what must be an unprecedented move they made a film from the recently cancelled show. This is something for which they should be applauded. I got my DVD set back in April last year, as soon as I could!
If you have never seen Firefly, watch Serenity and then seek out the Firefly DVDs for the backstory, if you have seen Firefly chances are that you will already be planning on seeing Serenity! The film has (apparently) been written to satisfy those familiar with Firefly and at the same time bring in those who aren't. Joss Whedon has said that the film was his toughest writing task to date. The buzz amongst those who have seen it is good - but I am avoiding reading reviews myself (as such there are no spoilers linked from this article, but you could try clicking on the tags below for up to the minutes news).