When I had the 150-500 I was very impressed with the images as long as you had good light and stopped down a couple notches. I mostly used it for backyard birding and got some really good shots with it hand held. It is a huge lens extended and the OS and HSM worked really well. It took me a while to figure out to turn off the SR on the camera or turn off the OS on the lens, because with both of them on it didn't seem to work as well as using only one.
I probably would of kept 150-500, but I with my DA*300 working so well in the backyard at 30-50 feet there was no reason to have both. If I was a true birder in the wild I think this Sigma would really be at home on a tripod and flash beamer setup.
Here are a couple of captures I took while I owned the Sigma 150-500...now it is at a good home in the great white north.
Red-belly Woodpecker (K-5 SIGMA 150-500: 1/640 sec, f 10, ISO 2000 at 440mm handheld)
Bluejay (K-5 SIGMA 150-500: 1/800 sec, f 8, ISO 2000 at 500mm handheld)