I agree. I'm really impressed that he could write a film that was both a sequel to a TV series and a standalone film. Enough detail was included in the film and enough attention to the characters provided that I could watch it with serious enjoyment. I think I mostly found myself confused by Inara and didn't even feel the emotion of Shepherd Book's death, but other than that, I could follow the film just fine.

Yes, I'm very impressed with Whedon's storytelling and dialogue writing abilities. He's a talented man who did a great job with Avengers.