I'm in the process of beginning a Wurld of Greyhawk Hackmaster game. So far, it's going fine. The characters can be a bit outlandish, but there's little harm in that. I'm enjoying it because (now that I'm older) I can give attention to some of the subtleties of the Greyhawk milieu while regaining some of the enjoyment of an old-style 1st ed. D&D game (like I played when I was a teenager (and before)). I'm finding that the two systems (HM and GH) can work fine together.