I've never played Hackmaster, but I've read through the books to try and get the flavor of it. To me, the core rules themselves seemed like a different 2nd edition. Sort of a different evolution of AD&D, incorporating the best of 2nd edition, plus other cool stuff, without adding a lot of the stuff I don't like in 3.x. But the over the top humor stuff like Blues Dragons and Gummy Golems turned me off a bit. That's after reading it for an hour or so in a store, so I can't speak from actual play experience.
But this new edition apparently gets rid of a lot of the over the top stuff, and seems to have improved upon many of the good elements they developed for the prior edition. They even made a lot of changes and additions, such as to Vancian magic, which actually look really good. They improved upon some of my sacred cows, it seems, and added a lot more cool stuff.
And in reading through this massive thread here:
I can't believe how much detail Kenzer gives about the Basic as well as the Advanced game.
I especially like Kenzer's attitude towards gaming in general, as demonstrated in his posts there. (Note: fans of 4e shouldn't expect to be overjoyed by his characterizations).
Due to the enthusiasm people have towards retro editions these days, and also due to Kenzer's experience in actually getting books on shelves, and most importantly due to the quality of the stuff they put out, I think Hackmaster is going to be something really big once all 3 core Advanced books are out. By that I mean way bigger than they ever were before.
I think they can end up in the pole position of the old school movement when the Advanced books are all finally released in about a year to 18 months.
Just a prediction.
Plus, check out the Otis cover art!