This is not for everyone; it's also one of those movies that you'll probably either love or hate (don't rely on reviews -- just watch). Its biggest fault is that a wild crescendo gives way to a less-than-dazzling finale. I was hoping for less of the typical Hollywood Happy Ending, since Altered States covers ground that most other films don't dare walk a mile near.
The psychedelic theme of the film is amusing, especially for those of us who have had some experience in that realm. While William Hurt's hallucinations are sometimes over the top, the imagery is nonetheless disturbing (almost frightening) at times. I don't know if the anthropologically diverse mishmash of the Mexican scene was intentional (or if they just thought nobody would notice), but at least it made for parts of the soundtrack being intriguing, such as the use of a European remix of a Tibetan Buddhist Evening Ritual (which probably would never be otherwise heard by most of us).
When I played this film for a friend, she really hated William Hurt's cocky, selfish character; regardless of which ones you like or dislike, you can rest assured that they are well portrayed by an outstanding cast. One of the fun things about this flick is that it all of a sudden becomes a sci-fi film somewhere in the middle -- with a truly original plot. Sure, it's a little bit silly, and it's too far out to really be believable, but so what? Even if sci-fi is not your thing, most film enthusiasts will enjoy this one if for no other reason than its thought-provoking script.