The Sigma 50-500 was a success for Sigma - it had very good image quality, in spite of the huge 10x zoom range, and an affordable price. The new version promised to fix the main downside of the old 50-500: the lack of image stabilization. I have tested the new 50-500 OS HSM on my 7D and I have compared it with its rival, the Canon 100-400 L IS USM.
Many thanks to DigitalFoto who loaned me the Sigma 50-500 OS and to Marco who loaned me the Canon 100-400 IS!
As usual, I recommend to give a look to the page Testing and reviewing a lens: it will help you to understand better this review and my testing methodology!