I've never played a game of HackMaster in my life, and yet I have the HM books with me every time I run an AD&D game. Yes, I shelled out the bucks for all of the Hacklopedia of Beasts books, too.
I see HackMaster as a wonderous sourcebook of optional rules that I can, or can not, apply to my AD&D game. The fact that the underlying systems are the same naturally helps immensely. But HackMaster is almost the Arduin of the modern day; a bewildering array of optional rules that can be bolted on to any AD&D game (or, I would think, OD&D game, too) with a minimum of fuss.
The things that I find of the most immediate utility are the monsters and the equipment descriptions and price lists. Every DM is always on the lookout for additional beasties to add to the stable, and Hackmaster does not disappoint in the least. There are ones that are of great use and could have come out of MM2, such as the Baboon Man. There are ones that are of everyday use and should have been included in MM2, such as new species of bat and beetle that fill in many gaps. And there are the way-out-whacky ones like the Great Red Marauding Beaver. The equipment lists are far more extensive than even those in Unearthed Arcana; we finally learn that a mandolin costs 28 g.p. and thank Pholtus that mystery is now solved!
The quirks and flaws lists are rightfully seen as the heart of the fun and silly side of the game, where you could end up with a magic-user who has a phobia about magic. But within reason, and being chosen rather than rolled (to avoid the possibility, for example, of a double-arm-amputee fighter), I think they could be of use. Certainly they are good guidelines for players who would like to include such things as part of their character development without the mechanism of gaining extra ability points in exchange.
There are a plethora of new spells as well, which could very easily make their way into a game via spell book or scroll. For those who think that a skill system is absolutely necessary, the one in the HM Player's Handbook is as good as, or better than, any I've seen (including the one in the Wilderness & Dungeoneers Survival Guides).
I approach HackMaster not as a game to be played, but a resource to be mined. There's a lot of good stuff of instant utility, and much more than can be used as guidelines with just a little effort. On the whole, if you can find the HM books on eBay or at your LFGS, you could do a lot worse than to snatch them up.