First, check out the Hero Games website. There's a ton of free material to give you an idea of what the system is about. Here's a link to an intro pdf:
http://www.herogames.com/FreeStuff/freedocs.htm
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Next, a little history. Hero system began as Champions, a superhero rpg. It was revolutionary in many ways, and did a great job creating the comic book genre. The rules, however, were in need of serious balancing. Other supplements came out, including for the wild west and espionage.
Champions II followed and was an improvement, but the system (IMO) didn't really come into its own until
3rd Edition. This edition cleaned up and consolidated a lot of the rules. Instead of separate rulebooks for different genres, everything was included in the main book. The worst complaint I can make is that the binding was lousy, so the book fell apart fairly quickly.
Fourth Edition (IIRC, called Fuzion) appears to have been a bit of a waste. I glanced at the rules, but never used the system. As another poster said, they weren't terribly well written. Ironic, since the idea was to make the system simpler...
Fifth Edition is the latest. The rulebook is fairly well organized and, best of all, has tons of examples of powers, rules, and campaign ideas. The system also has a lot of support - if you're going to use the system for superhero gaming, definitely pick up
Champions, and consider
Millennium City (a city book) and
Conquerors, Killers, and Crooks (a villain book). There's also quite a bit of free material on Hero Games website. If you want a character creation program, Hero Designer 2 is available - I'm still not completely sold on this product, but for now I'm forcing my group (including me, the GM) to use it, to give it a fair test.
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Last, for my take. I agree that Hero System isn't for everyone. If you're the type who loves designing new mechs in Battletech, or always wanted to design your own ships in Star Fleet Battles, or enjoys spending hours designing characters in your favorite rpg, you'll probably like this game. It's perfect for the person who likes to tinker under hood, so to speak.
The system is incredibly flexible, and has the potential to create virtually any character concept you can imagine. No multiclassing to have your character fit your concept - just design him/her the way you want. Drawback: the system can be broken in so many ways. I recently had one player suggest (as a joke) creating a character with 60 points in Presence. The way the system works, he would basically walk into a room, make a presence attack, and every enemy would fall down in awe. My response: Uh...No. Bottom line: the GM has to exert considerable control over character creation. Having mature players who don't go overboard helps.
Combat is very cinematic, but favors those who approach it tactically. A player used to a more free-flow, storytelling environment will likely be lost. I prefer tactical combat, so I favor this system. Now while combat is tactical, flavor isn't lost. Nothing is more fun than knocking the super-villainess through a wall (or two) while fighting on the 30th floor of a skyscraper - the thud when she hit the ground below was sooooo satisfying.
And as others have said, it's much easier to customize a campaign with the system, than with (for example) a more rigid system such as 3E. If you want a low-magic D&D campaign, you'll need a lot of house rules. In Hero, you just lower the point totals and design a low-powered magic system. In effect, you don't change the basic rules of the game so much as choose which options to use/allow. Of course, that flexibility comes at the cost of forcing the GM to design an awful lot from scratch. Remember what I said above about spending hours designing stuff? Count on it.
Finally, IMO the system is better for superhero campaigns than fantasy. I've GM'd and played in both, and I like the system - a lot. In fact, I gamed with a group in the 80's and early 90's that didn't play D&D at all, because we spent all our time with Champions and Fantasy Hero. But there's much less work for the GM in a superhero campaign, and the "feel" of the system seems to fit that genre better.