Well i'm enjoying the new glass. Sharper then my old 70-200mm f2.8 IS, no surprise really. But it is also sharper then my 200mm f2.8 II wide open, with as creamy bokeh. I'd comfortably run it head to head against a 135mm f2 @ f2.8.
I'd say this is my first zoom that outperforms the primes that i own in the same range. Accept for the 85mm f1.2, but that is purely down to DOF advantage. The AF is lightning fast and the IS is class leading for any Canon lens i've used. I guess it should be, because it's the latest generation.
5D II, f2.8, out of the cam, no sharpening. Not an exciting shot, but a reference shot to show the IS advantage.

200mm, 1/25th sec IS on 100% crop, no sharpening

Same settings but no IS

I would never of got the same results out of the mkI. I never handheld that below 1/100th at 200mm.