Was lucky enough to see "Serenity" a few weeks back, and I can say that it really is one hell of a film, dramatic, scary, funny, it has everything, along with great acting, a good story, and excellent special effects.
People who loved "Firefly" should love this movie (if they're willing to accept that not every single thing they loved from the series can be fit into a 2 hour movie (be patient, if this makes Universal money they're signed up for 2 more movies)). People who never saw "Firefly" should give this movie a chance, I thought that everything you would need to enjoy the film was provided, there are just a few lines and references that they won't get like the "Firefly" fans will.
Can't wait till October 7th to see it again.