This is vintage Glass. If you like "Koyaanisqatsi", you should like "Kundun". Like all of Glass's work, this one takes some time to get into, but once you've heard it, it becomes compelling, hypnotic, even addictive.
One of the reviewers below complains that this music isn't Tibetan enough. This is like complaining that Beethoven doesn't use Flemish folk songs enough. If you want traditional Tibetan music, buy some. If you want Philip Glass, buy "Kundun".
It's true that the music is vaguely reminiscent of "Koyaanisqatsi". This is not necessarily a bad thing, "Koyaanisqatsi" being one of Glass's greatest works.
Some of the music is also reminiscent of Bernard Herrman's score for "Journey to the Center of the Earth". Since Glass is a Minimalist instead of a Romantic like Herrman, however, don't expect to hear the great bursts of emotion you find in "Journey to the Center of the Earth". On the other hand, Herrman followed the action of the movie so closely that his score sounds like a series of unconnected pieces. "Kundun" is far more unified, and you feel like you've heard a symphony when it's over.
One of the reviewers complains that the orchestra doesn't seem to contain many Tibetan instruments. I wonder how many Westerners can recognize Tibetan instruments when they hear them. If you look at traditional non-Western instruments all over the world, you find the same general themes over and over again: flutes (like the Andean pan-pipes), horns (like the Tibetan horns heard in this music), drums, and stringed instruments (like the Chinese biba or the Japanese koto). It takes a sensitive ear to hear the difference between one of these instruments and the Western equivalent.