Let me throw a little more color into the picture...
The truth is (I own the HackMaster Player's Guide) that Hackmaster is at its base the 2nd edition AD&D rules, with many variant rules added on, and including some material for humor value. Many variant rules are very fun and palayable (the talents you can purchase at 1st level, the percentile method of incrementally improving ability scores, the fact that bonuses start at 13+ instead of 15 or 16 on ability scores). Many other rules seem to be unwieldy (the small, medium, and large weapon damages for each weapon, the honor system, the "dibs" rules on treasure, etc.).
It's got some humor built in, with a built-in assumption that players should not take a game system deadly seriously (it is meant for fun, after all), and as proof you have such items as battle mages (who put their spells "on line") and berserkers (who can't go a day without combat or risk penalties) as character classes, and races such as Gnome Titans and Grunge Elves, and Pixie Fairies to add some fun in the normally staid diet of Dwarves and Elves. Plaster on things such as "Skipping Betty" Fireballs, Spells that explode your eyes, and low-level raise dead spells that give you a random number of deformities.
In short, if you have a character that made it to high level, it's a freakin' accomplishment, not just because your DM took it easy on you and let you get there because the story wouldn't be told without you.
Fun game, to me it's massively overpriced for all the supplements, but if 3E and d20 seem to you and your group to be too little fun and too far away from those great days of chaotic gaming with friends in a dorm or basement, then it definitely calls to you more than d20 does.
Now I'm sure a solid lover of Hackmaster will feel free to correct me.
