As much as I enjoy the more creepy varieties of anime horror -- and Boogiepop Phantom is no exception -- this anime is about as difficult to understand as any anime I have ever seen. Boogiepop Phantom manages to keep my attention most of the time with its dark themes and its fair share of horrifying scenes, but I cannot claim to understand it well enough to explain the plot to anyone with any kind of confidence.
The opening music is really good, and I also like most of the music themes chosen to play throughout the series. There is a lot of electronica as opposed to more cultural-specific music, which can be harder for us Americans to appreciate.
The color is interesting. You are unlikely to notice any bright, primary colors; the whole series has a sort of washed-out look, which seems appropriate for the horror theme.