One of the very first albums I ever owned. It was a gift. Saw the film at the theatre in 1968 as a kid and was stunned. Still am to this day whenever I see it.
The Ligeti soundtrack was unconventional and perfect. Apparently Ligeti was stunned Kubrick wanted to use his music. He thought no one knew of him.
I think listening to Ligeti now is interesting because his music does sit in the greater context of 20C classical music, however at the time it seemed like it came from another planet, thus appropriate for 2001 :)