Just got my lens this morning and so far I am very impressed. I came from the Sigma 70-200 non-OS and it always performed great, but I really wanted to have an IS system so I could use the lens indoors. In the past I have rented version 1 and was generally pleased, but there was a little bit of softness at 200mm and f/2.8 (not bad, but my standards are high shooting with the 17-55 and 400L).
Compared to the Sigma, the Canon has a more robust build quality and the focusing system is a little bit quicker and feels more precise. The hood is much better (Sigma hood scuffed really easy). The zoom rings are about the same, but my Sigma zoom ring actually had a nicer dampened feel to me. Basically the same in real world though. Image quality is definitely a step up at all focal lengths and apertures and the Canon renders an image with better pop. I believe this is due to slightly better colors and less CA wide open. I did prefer the Sigma tripod collar. It was removable with the lens mounted on the camera and easier to use. Just a minor nit.
Compared to IS version I, I like the new hood a lot better. Weight is very similar, but it does feel slightly heavier which is no big deal. The zoom ring feels a little bit more stiff to me. I don't know if this is common or if mine is stiffer than most (?) Image quality is better across the board and the colors from the Mark I had a more muted look. Bokeh seems to be similar to me and there isn't enough difference to worry about I don't think.
A couple of test shots. (miserable weather, sorry).
ISO 1600- 1/15 @f/2.8
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ISO 1600- 1/20 @f/2.8
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ISO 1600- 1/15 @ f/8
What it looks like outside today
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