As of System 9.0, I've been a rapid upgrader. I had 10.0 installed the day it shipped, and I've installed every major version since then within a week or two of its release.
10.x.x versions, on the other hand, I'm more cautious with; I have a tinderbox machine that I usually try them on, but I never upgrade my home machine until watching MacFixit and the forums here has me convinced they're not going to do anything disasterous. I've skipped a few as a result.
I will note that, were Apple to ever release a real clunker of a major upgrade, I wouldn't blindly install it, though I'd probably get it on something if just to familiarize myself with it. I do not like Windows XP, for example (vastly prefer 2K pro), although I'm using it on one machine to get used to it.