I've played and run Hero System since shortly after its release. For my home game group, its our go-to system. And we don't ever play Champions with it!
Years ago, when I joined what is now my home game, they were playing Fantasy Hero in somebody's homebrew world. When I got a chance to run, I ran a Hârn Hero game which used the FH rules as a basis, but I made major modifications to increase lethality and built the core Hârn spells in the FH engine -- it gave it a very different feel. That campaign ended up going on for over ten years in three "seasons" of campaign play.
During the season 1 and 2 Hârn game hiatus, another buddy ran a game where everybody played whatever kind of movie or TV character they wanted, in a hodgepodge, kitchen-sink sort of setting. One guy made James Bond, another made a low-powered superhero sort of like the Thing, I played a two-fisted pulp vigilante, and so on. It turned out we were all characters being played by patrons of a Virtual Reality Arcade. Cool stuff.
After my Hârn meta-campaign ended, I ran Traveller Hero. Again, I rebuilt the Traveller-style psionics in Hero, and was able to capture the flavor of how it felt to use Classic Traveller psionics, but in a system that didn't have different subsystems for everything (my one real criticism of Classic Traveller -- everything uses a different subsystem). That campaign lasted even longer than the Hârn one did.
In the seasonal hiatus of Traveller, another friend of mine has been running a Battletech Hero game for us. He uses some of the classic Battletech mech rules, grafted onto the Hero System chassis. It worked pretty well, and fights didn't end up taking as long as you might expect from my previous sentence.
My next campaign is going to be an 1870s Mashup game, based on
this meme from David J. Prokopetz. Everybody has to play one of: a gunslinger, a samurai, a gentleman thief, or an old, retired pirate; but with Hero System, there's still plenty of character you can put into them to make them very distinct from other characters using the same archetype. I've a few ideas for what's going on in the setting, and I'm playing every single one of those cards so close to my chest they might as well be inside my ribcage...