Here are a few quick observations.
1. Hard to believe, but it is sharper than the old 500 which I regarded as the best birding lens ever made.
2. Image stabilization is very good indeed. Not sure about the 4 stops but it is very good. Equaling the new 300mm f2.8 ll which I have had for the last 8 months now. One thing bothers me. The noise of the stabilization when it starts and stops makes a loud click. My 300mm ll has a similar sound but much quieter I am going to ask Canon factory service about it next time in BKK and will give an update there.
3. Also when purchasing the lens out of the States. I get a 2 year warranty? I spend a lot of time in Asia now, so decided it was worth the purchase here instead of America? Personally I think just another way Canon is putting the screws to the American consumer. Two years instead of one year is a big plus for me.
4. I purchased it for the weight savings mostly. And I will say here and now, it is a god send for me. The extra sharpness is just a bonus. It is very easy to hand hold. All the pics here are hand held.
At the moment I am on the road heading to Fang Thailand for a shoot of a recently found bird in the mountains of Doi Lang. So when I get back to my apartment in Phechaburi Thailand. I will update this and give a detailed view of the new lens.
This firs pic is wide open F 4, ISO 100, 1/640, Canon 7D.