I was reluctant to watch it because the series ended, in my opinion, nearly perfectly, but wow, they actually managed to make a sequel that lives up to the show, and is open ended enough to continue in another movie.
It continues to work as both a strong story, and as a SUPER dark deconstruction of the genre. The trippy visuals turned me off at first, since using surreal settings to justify your story not making any sense is one of my biggest pet peeves not only in anime, but in storytelling, but the movie not justifies the visuals, but tells a coherent story using them. It's a gorgeous film, and it tells a good story.
Even the basic premise subverted my expectations for the series. What I thought the movie would be was the Magical Girls fighting the Wraiths seen at the end of the series. Without spoiling... It's not that.
As for the ending... I'll have to see where they go with it in what I can only assume is a planned sequel.
Overall, excellent movie, any fan of the series should really watch it (and if you haven't seen the show, go watch that!).