The excellent 'Serenity', a feature film based on the equally excellent 'Firefly', is available quite cheaply at Amazon right now. As I write it is below the six quid mark. I thought 'Serenity' was a great film, it was written to both please people familiar with 'Firefly', as well as introducing new viewers to the world.
It didn't get the recognition it deserved at the time, which was a shame, but it's well worth picking up on DVD.
Given the choice, I'd recommend that people watch 'Firefly' before 'Serenity', as the latter ties up some loose ends in the former.