While Heroclix aren't an RPG, they are an extremely awesome game unto themselves. Concerning RPGs, they make for fantastic miniatures.
What's neat about them is the variety that is available. There are hundreds of characters that have been Heroclixed. But even the same character has many different versions of themselves out there, all historically based in the Marvel Universe. Not to mention the THOUSANDS of Heroclix that are generic (thugs, cops, etc) and from other properties, making it easy to integrate, say, you favorite DC heroes into your Superhero RPG. How cool!
What's even cooler is that they have made a ton of larger Heroclix more or less to scale. Bringing out a giant Sentinel or Galactus is more impressive when it dwarfs all the other figures on the board.
So Heroclix have a totally awesome secondary purpose - supporting your RPG adventures.