Great question.  I stumbled across one of these sites a few weeks ago myself and had the same thought - if it's something that people have had some experience and success with.

I don't think I'm ready to put anything on one of the sites.  I'm a newbie trying to find my style at this point, but I was cusious and thought it might be a goal in the future.

I would love the hear from someone that could say, "Yes, I use these sites and this is the primary way I sell my work" or is this something that can be a ego boost and get some pics our in the marketplace but will never amount to anything financially significant?  I'm not expecting any photographers are getting rich off sites like this, but I wonder if a high-demand pic on one of these sites represent tens of dollars, hundreds of dollars, or more?

I guess ultimately, like Onslow says you would have to know the market and tailor pictures that would be useful for business publications and what not.  That doesn't really interest me, so I suspect I'm out, but I'd still like to know more about it from people with experience.

I guess this was just a really long way to go to say "ditto" to your original question.  Sorry.

David.