I bit the bullet and bought this lens and the 2x II converter for my Africa trip. Figured spending so much to go there demanded a comparable lens. My wife's still not happy, even after seeing the results, which she loves. She just can't see payng so much for anything!
The most amazing thing to me is the speed with which it focuses--wow! I have a nice little stable of Canon and Tamron lenses and this just outperforms them all for speed of focus. The sharpness is right up there with my 100 mm macro, and I can't tell much difference with the 2x II converter.
It handled wonderfully for handheld shots from the Land Cruiser at Chobe NP, Botswana where a tripod would simply not have been an option for me. The IS is incredible for shooting from the vehicle.
This is a no-brainer lens: go buy it!