OK, so I just got this lens this morning. I played with it for a few minutes and already I regret getting it.
I previously had a Tamron 200-400mm and Tamron 2x extender, both of which I gave to my Sister for Christmas. I have a 300mm f4 and canon 2x extender. I was happier with the IS on my canon lens, focus was faster, pictures were sharper.
Here is my gripe with the 200-500mm. Pictures looked good, color was improved dramatically from the 200-400, but the canon 2x teleconverter did not fit on it! I was really hoping to get a 400-1000 f13 lens.