I've never seen the third film, but the second disappointed me. The first film, however, is terrific, and hopefully will get a worthwhile HD transfer.
I liked the third, but then, I liked the second (mostly because Glass' music is excellent for both). The first really is the one of the three that's superlative, though.
Does anyone know if a fourth film is in the works? I don't know what it could possibly be about. Ponyqatsi?
Don't think so, but this year Samsara, essentially a sequel to Ron Fricke's earlier Baraka (which in turn owes a lot to Koyaanisqatsi was released, and it's pretty good. Neither of Fricke's films have Philip Glass, unfortunately, but can't have it all.