I'd recommend the Phantom of the Paradise for any fan of glam, goth, 70s rock excess, the music industry in general, De Palma's films, sophisticated pop harmony, horror, Paul Williams, or just good movies that are a bit different. In many ways it's like a cross between Body Double and Rocky Horror, retaining the best traits of both. Williams' songwriting is very interesting, much better than his more famous songs for Bugsy Malone, The Muppets, Streisand, or The Carpenters. It's a much overlooked treasure of music, dark humor, and decadence.
Phantom of the Paradise is also a must-see for fans of Stevie Wonder or of electronic instruments. Bob Margouleff and Malcolm Cecil's legendary modular synthesizer T.O.N.T.O. ("The Original Neo-Timbral Orchestra"), which they used for all of Wonder's albums from 1972-75, makes a guest appearance as the instrument the Phantom uses in his suite. It's a beautiful instrument visually, as well as aurally, and it's nice to see someone actually playing it.