Well... they favor exclusives several ways. More uploads, more money per download and faster approval times. A couple of intangible ways are that the bar for approving an image is lower for exclusives and they show higher in "best match". Sean L. may tell you otherwise, but a bunch of us on the Micropayment list proved it about a year ago...

Jeez, I feel like an old timer here. I started in 2004 but still have a fairly small portfolio. I remember when it was like 10 pictures a day for everyone and as time goes on it gets smaller and smaller. I rarely fill up my weekly quota, but recently I've been hitting it and wish I could get more in.

I ran into the same problem last year that epixx is running into. I went through this period from March to September where after making best month for like a year straight, I ran into a brick wall. Then a year ago, things picked up through May of this year. A lot can be attributed to the seasonality of the stock business. The one thing I've found is that IS changes it the search engine algorithm many times. One image I had went to flames in a couple of months and about a year ago something changed and I've had like two downloads in a month. That took a huge bite out of my monthly downloads.

The one thing I find with IS is that it takes a while for images to start getting downloads vs. SS or DT...

I submit to iStock first just because I started there three years ago, even though they have the most cumbersome upload process. Even worse than Alamy. If they reject it a vague reason,  I'll upload it somewhere else. If there is a technical issue, like logos I'll fix it and upload again. Then I FTP large batches to SS and DT and so on... No reason I couldn't start with one of the other agencies but in my experience IS has been the toughest so if it gets through them it usually gets through the others.