My relationship with corbis started when they purchased Solus images, a small agency that sold very well for me. Everything was fine for a few years, but last spring they changed everything around, and essentially cut my earnings there by 90%.
My observation on the whole enterprise is that they don't seem to have a very capable management. They seem to always be a year or two behind getty, and are not very good at playing catch up.
The Bill Gates money has not helped them, but it has made it possible to do poorly and still stay in business.
I would also observe that my design clients almost all prefer to deal with corbis rather than getty. Many of them absolutely hate getty. Yet when it comes time to actually buy the image, they end up at getty. Getty has better images, a better website and more depth, unfortunately wrapped up in a culture of arrogance.
My point would be that if corbis is open to you and getty is not, go with corbis. But don't expect miracles.