I'm not an iStock contributor, and I'm still fairly new to this game.
I joined Shutterstock in July, and I have a couple files that have already been downloaded 200+ times. Obviously they are successful based on number of downloads, but revenue is mostly $.25 for each download. It's still good, and I'm happy with my results there. The first Microstock site I joined was DT, and it was my focus for the first few months of my microstock career. However, in my first month at SS, I made more than my first 6+ months at DT.
It's interesting to me how one file can be crazy successful at one site, and not sell at all at another.