Altered States is an extremely bizarre movie that should ideally be watched in an altered state. Strange religious and evolutionist imagery, cut scenes that set the standard for this type of movie (i.e. drug/psychedelic film) and occasionally overpowering audio seem to characterize this 1980 film.
Though most people will probably strongly dislike this film, a small minority might greatly value its artistic, ridiculous and surreal feeling. Sometimes, it needs to be taken with a grain of salt. I recommend this film for anyone that liked Requiem for a Dream, Anarchist's Cookbook, Waking Life, Fear and Loathing in Las Vegas and similar films.
The plot is a little thin, given the rather repetitive nature of the "experiments", though the concept is certainly interesting and could be adapted for a more contemporary film, given the film's age.
By and large, I recommend this film, though can't guarantee everyone would feel the same way. Ultimately, don't take this film too seriously.