It's hard to believe I was ready to get rid of my cheaply obtained Sigma 150-500mm lens only a week or so ago. I had been using it with my EOS 7D and SL1, both APS-C crop sensor bodies and the results were unacceptable. Once I used it with my full frame EOS 5D Mark III, however, the results were, and remain, useable.
On Peebles Island this morning, the eagles were boring and during the half hour I was there one sat on the nest and the other on a branch nearby. I decided to do a test of the image quality other than in the center of the image circle of the Sigma lens.
There are certain truths to getting decent images from this lens. You need a full frame body. You need to shoot at f/11 which means you need to have good light. Even with image stabilization and a monopod, you need at least 1/750th of a second to get sharp results. But, I wouldn't hesitate to go up to ISO 3200 with the 5DIII as long as I'm not doing HDR processing. Today's images were f/11 at 1/750th and ISO 800.
Here are the original photos with 100% crops. I'm not noticing a lot of difference. The vignetting (light falloff toward the edges) in the uncropped shots is apparent, even at f/11. This isn't really a problem for the wildlife or sports purposes for which I'd be using this lens.
The bottom line is I'd recommend this lens for anyone using a full frame body that wants a 500mm lens without shelling out a lot of funds. Used, this lens can be had for around $500.