Never played it, although I have some of the stuff. I DMed in Forgotten Realms, dropped that for Planescape (which quickly became a homebrew-planescape, and then just homebrew).
There is something about Greyhawk which irritates me, I think it's the NPCs. They felt like thinly veiled character insertions and I hate that. I had similar problems with FR, but not to the same extent; it makes me feel like I'm playing in someone's pet setting and not a real world.