How successful do you feel mainstream super heroes have been entering the world of prose? Do they only succeed when they have a major storyline tie-in such as Kingdom Come or Identity Crisis?
As a writer with my own collection of super hero short stories (SHOW ME A HERO), I really wish the market for original super hero fiction was a better one. Sadly, I've found that unless there's a connection to a popular existing property like one from DC, Marvel or even a best-selling indie, the books may do okay by indie standards, but won't hit the mainstream with much impact.
However, that said, some of the best super hero fiction I've read comes from indie publishers, and books like Van Allen Plexico's Sentinals series and the books from iHero Entertainment stand among the best I've seen.