Saw this last night, very entertaining. Funny, original, and unexpected, exactly what you'd expect from Whedon.
A lot of blink and you'll miss it references and jokes, so I think I will need to pick this one up on Blu-ray when it comes out and give it a few more watches. I have a feeling it will take a long time to get old.
After such a long delay getting to theatres and what will probably be a modest take, i'm hoping this gets to the home video market sooner rather than later.