Clapper,
When I wrote this is was most thinking of iStockPhoto. While the are the most successful agency, the amount of time and effort it requires to submit is not worth the income. I still make almost as much with them as Dreamstime or Fotolia, but they aren't worth the time anymore.
Most others are o.k. I wish Fotolia's site wasn't so slow, that would make me happier, but they are not near as picky as iStock.
Dreamstimes changes their prices to much. I don't always understand why. Sure we make more, but simple supply and demand says we'll sell less. I think I am balancing out a little tiny bit higher this time, we'll see...
Sorry about the grips, hope this answers your questions. If I were starting out now i would stick to Shutterstock, Dreamstime, Fotolia and BigStock. Support other sites if you think they are unique and may make it (I like Lucky Oliver).