Hackmaster's Player's Handbook and GameMaster's Guide carry disclaimers that they contain parody and satire, but the game is, in fact, playable, as can be attested to over at Kenzerco's HackMaster forums. The parody and satire are mostly of players and playing styles which have developed over the past 30 years (see the Knights of the Dinner Table comic, some strips of which are available online at Kenzer's site). In that regard, the parody and satire may not make a lot of sense, or hit as close to home, to someone who is new to the hobby. As has been said above, the game is an amalgamation of 1st and 2nd edition Advanced D&D rules, with a number of new, original additions.
Personally, I love the books for the game, especially the early releases: Player's Handbook, GameMaster's Guide, the 9 volume Hacklopedia of Beasts, and Little Keep on the Borderlands. The game has also spawned some really neat ancillary products, such as the combat wheel (which calculates combat figures) and my all-time favorite RPG game add-on, the GameMaster's shield, which puts all other gamemaster screens to shame. Various adventures were released, most of them "hacked" (as in converted to HackMaster) versions of classic first edition modules from D&D's early years. Some of them are actually pretty nice revamps of the originals (Lost Caverns and Temple of Existential Evil being two of my favorites).
The setting for the game, Garweeze Wurld (which I've always assumed is "Gary's World," as in Gary Jackson, the fictional creator of HackMaster) is available in pdf form on Kenzer's site, in two separate pdfs.
Overall, while I love the whole concept, I have to admit that the game is very rules-heavy. It threatens to collapse under its own weight (which is actually part of the parody, in my opinion). Much of the humor is based on long-time gamers recognizing some of the tropes of early D&D, including the "Gygaxian" writing style. The books are very densely written, with a lot to absorb. I find them fun. Your mileage may vary.
