There are many reasons to like HackMaster. The character creation is fun. The monsters - the ones that aren't standard AD&D critters - are both amusing and playable (the Swack-Iron Dragon is a pretty deadly critter). There are lots of random generation charts (I like stuff like that) that can be used in any game, but fit the feel of HackMaster. But, rather than make a laundry list of merits, and to avoid any kind of qualitative comparison between HM and any other game, I'll tell you the best reason to play HackMaster right now:
The GM screen.
If you don't immediately want to play (or better yet, DM) HackMaster after taking a look at the GM screen, then HM isn't the game for you.