In college, I knew some guys who were really into HackMaster; it seems like its all a lot of them played for something like two years. I did play with them from time to time in various campaigns started by various members of the group, and while we did have some fun it wasn't really my thing. At the time, I chalked it up to Hackmaster being essentially a work of parody, but in retrospect I think it was more the chemistry and gaming style of some elements in the group, rather than HM itself. Some of my earliest D&D book learnin' was the 1st edition AD&D DMG, and that actually had quite a bit of parody in it. (Or at least some particularly "zany" stuff) Of course, the humor in HM is a retrospective send-up of "old school gaming" rather than contemporary gaming humor...
...or is it? HackMaster is completely functional. The game mechanics have been revised with a degree of effort that goes far past just making jokes. The skill system isn't bad, and sort of fits with the way thief skills work (rather than all skills in the world working one way and all thief skills another like in the RC) The new classes, for the most part, work; Knight Errant is pretty parody-ridden, and all the new classes have a bit of silly written in, but they could easily have all the silly bits excised and then dropped into an AD&D1 or OSRIC-type rules hack. If the skill system had all the "joke skills" (Advanced Looting, Wuss Slap, etc) removed and/or renamed...there's a functional non-parody game waiting to be had that's sort of....well, I hesitate to call it a specific edition, perhaps AD&D(1+2), a sort of third edition that is a roll up of the previous two editions plus several notebook's worth of house rules.
About a year ago, I bid on the HackMaster Player's Handbook on Ebay. I'm not sure why I did it, but when it arrived I immediately shelved it, occasionally taking it out on random afternoons and reading bits and pieces, mentally removing the blatant parody or reworking it into something useful. I realize now that there is a part of me that wants to remove the useful/neat bits of HackMaster and graft it into AD&D/OSRIC. I would also like to get my hands on all the sub-classes they invented, but I'm not willing to drop coin on the fighter, thief, priest, and magic-user books. If nothing else, HM is a neat example of what you can do with your game when you tinker it to your tastes.
A parting thought on HackMaster...I do recall the times when I had fun playing, rather than being frustrated, were the times when it was being played as a straight-laced old school fantasy game and not a parody or as a just-for-laughs beer n' pretzels type game. Those guys who were into it...for some of them, it was their third edition, and I wish I'd had a better understanding of that at the time.