Madhouse's Boogiepop wa Warawanai: Boogiepop Phantom is a puzzle to be solved by the viewer. The chronology is non-linear, many important events are only briefly shown and, if you haven't read the light novel Boogiepop and Others, you'll be without context for much of the story. Still, many people (myself included) first watched this show without having read the light novel so don't feel as if it's an obstacle, it's just something worth bearing in mind. Even with the context of the light novel, it's still something of a labryinth; the closest thing I'd compare this anime to is Serial Experiments Lain (Triangle Staff assisted on both and the great Kaori Shimizu voices the titular characters) but Boogiepop Phantom is by no means a derivitive work.
At its core, the story in Boogiepop Phantom is about adolesence, society, our place (or, lack thereof) in the world, identity, trauma, humanity and morality. If you couldn't already tell, it's an anime that invites you to ask questions, questions which extend far beyond the scope of the story. You won't pick up on everything the first time or even the second: this isn't a show that you watch once and forget about, it slips into your psyche.
The audiovisual side of things is just as unique as the story it tells. For the majority of the show, a heavily vignetted, sepia-esque filter is applied which gives it an unsettling and unmistakable look. It also doesn't shy away from gore. Sounds are garbled, distorted and strewn about to create some of the most disturbing audio cues in the medium (it's also, to my knowledge, the only soundtrack where you'll hear drum and bass and Wagner).
If you enjoy Boogiepop Phantom, supplement it with the light novels if you haven't already done so. They enhance each other and the Boogiepop universe is fascinating and worthy of a deep-dive.
