This movie was an excellent side story to Holic: creepy yet comedic, structured yet whimsical. If you haven't seen it you should, and of course, this movie is recommended only for those who already know the characters of the series.
The story moves quickly and I enjoyed the vivid depictions of the collector's 'meeting' place. The art was pretty well done. If you're not a fan of CLAMPs 'noodle people' (quoted from TVTropes), then you shouldn't watch this since the anatomical correctness of Holic is questionable at best (especially Yuuko's plunging neckline). But why would you see this is if you weren't a CLAMP fan anyway? ... *ahem* ... Going back to the storyline, it isn't super predictable but it is interesting and original. I enjoyed the structured mystery-novella plotline and the interesting subject which points back to a larger theme, so if you're looking for something typical yet different (opposites, I know) then Midsummer Night's dream'll cover you.
I have to say the music didn't particularly move me although I DO remember it, which is a rare occurrence for most movies. Memorable music for me is a must! To be honest, the only thing that stayed in my mind was the instrumentals from all the more demure creepy moments of the film.
So if you're questioning why you should buy this movie, think of all the Holic goodness and the high rewatchability factor! I've seen this movie 3 times and I'll definitely watch it again! ^_^