Win.
It’s a tricky one. The film had to pitch to both the diehard fans and people who’d never seen it before. So basically you get fifteen episodes of the show respun and condensed into the the first five minutes of the film. So it kinda works, but it’s a bit like watching Empire Strikes Back having never seen Star Wars.
Consequently, a) you’ve got a lot of spoilers going back to Firefly after having seen Serenity, and b) you’ve not got the same emotional attachment to some of the sideline characters in Serenity. When one of them dies in the desert it’s kinda “meh, next” out of context, whereas to someone who’s watched Firefly they’ve just killed off one of the most intriguing characters in the show.
Serenity is a reasonable stand-alone film. As Firefly’s swansong however, it’s gorram shiny. Firefly has a cult following for a reason, it’s one of the best genre shows ever created IMHO.