Serenity

Non-spoiler review:

Wow. Leaves one feeling kind of stunned.

For the folks that have seen Firefly:

Definitely a different feel. Less humor/snark, and characterization, more focused on the action/adventure. That said I think it's the best action/adventure sci-fi movie I've seen in, well, ever.

For the folks that have not seen Firefly:

I think the movie will stand on its own, but it will be enhanced by seeing the series first. There is very little exposition in the movie, so you are missing alot of the "who are these people and why should we care about them." You don't really need to know that to enjoy the movie, but it will be alot more powerful if you do. If you don't want to spend the time watching the entire series, try to see "Serenity" (the pilot), "Out of Gas," and "Ariel" for a crash course on the bits most relevant to the movie.

Regarding certain spoily things:

Joss Whedon is a bastard. Please no spoilers in the comments.