I love GURPS supplements; I am annoyed with the system.
I used to play it back in New Hampshire with a guy who worked on a couple of the supplements (Stephen O'Sullivan, very nice guy). The problem I found with GURPS is that in trying to be everything, it ends up being very little. This is what initially turned me off to "universal" systems. To this day I have yet to find a system that works with all genres, no matter what people try to sell me. I find the game far too crunchy and far too driven by the "hunchback albino midget" syndrome -- taking too many flaws to get the character you want to play.
That being I ran in a very good GURPS Tekumel campaign in Santa Barbara, CA, but that was mainly because the GM was very cool and we ignored a lot of the rules.
Still, SJG has put out some of the greatest gaming resource material on the face of the planet. In various supplements I have found fantastic quick-n-dirty info on: Classical Greece, The Book of the New Sun, modern arcane conspiracies, the Aztecs, Dynastic Egypt, futurist views for the next couple hundred years of human development, 17th century Europe, the Golden Age of Piracy, fairytale Russia, and an alternate early Victorian London populated by goblins... Each of these supplements has proven to be a treasure trove of facts, ideas, concepts, and gaming thoughts at a reasonable (for rpgs) price.
So currently I own over a dozen GURPS supplements, but no longer own the core rules.