well it is right up there for me with producing site 4,5 and 6
dreamstime, istock, and shutterstock are clearly the best, then 123RF, Bigstock, Fotolia and stockXpert are all competing for 4th place. So i suppose it is up to you wether it is worth it. If you want to add just one more site i think i would choose 123RF or stockXpert first as it takes bascially no effort to upload to these. Bigstock is a little more work, and Fotolia is the most work.