istock has stopped selling videos for me. I had the free video of the month in January, which gave me excellent exposure, but I didn't sell a single file. The same happened to the guy who had the free video for February.
Maybe if you are exclusive it is a good place to make money, but my rumour mill keeps telling me that pond5 and SS are the best sites for video.
There are also many other small sites, but I still have a tiny port. so I want to focus on just 2 or 3 sites until I have more experience. And pond5 and SS have proven themselves to be reliable.
The only thing I dislike are the visible download numbers on Pond5. I hate it when sites encourage people to copy my work.