What would possess anyone to use any other rules set once they use HM(in any of its incarnations) The magic and religious spell rules make awesome sense, its extremely difficult to min/max characters, and super easy to adapt the rules to ones specific campaign. End Rant.
Bah, I say, bah!
While I love the world of Harn, I have never understood the love of the game rules. They work, they are consistent, and they are better than some other systems, but I would never use them as my first choice. If nothing else, they are tied too tightly to the world, and to one style of play. If you want a system where magic is non-existent or rare, combat is discouraged and very deadly, and gods might exist, but never make an effort to prove it, then HM is a good fit. Try to do anything else, and IMHO, it starts falling apart. I love Harn for the maps, and history, and detailed settlements. I love that canon and fanon can easily handle a campaign with no magic or heroes, and a campaign with heroic swordsmen and a mage or three. The world can handle a diversity of campaign types much easier than the rule set.
IMPH, magic exists, using it in public is socially, but not legally prohibited, and the gods have been known to manifest on occasion. These things are rare, and they don't happen as often as the general public (a cowardly, superstitious lot) thinks they do. Fighting gargun, barbarians, Orbaalese pirates, and the land-grabbing lord next door happens very regularly. Lots of people die, but a rare few become very good at it, and they don't break a sweat when confronted by a couple bandits on a trail. Harn can accept this type of play with no change to speak of. Harnmaster might handle the combat, but it doesn't handle it well. IMHO, Harnmaster just doesn't like heroic play, and makes it too hard to do.
All that said, I don't think I've done any fantasy gaming in the last 7 years, and no real roleplaying in 15+. At this point, I consider myself a storyteller / world builder, and most of that is done within my version of Harn. Unfortunately, I complete projects even slower than Columbia Games. Maybe someday I'll finish my corner of Melderyn and it's history, past and future.