Much sharper than I expected, at both 70mm and 300mm, as well as wide open. The new IS is just AWESOME. I got good very useable shots at 300mm at 1/50 of a second. Focuses faster than I expected, and the focus is VERY accurate. Almost no missed focus.
Again, the 3rd gen IS is fantastic. I even got a useable shot at 100mm at 1/8th of a second. This lens kicks butt, and I am much happier with this than the 70-200 F4 L that I had for a few days. It had terrible focus accuracy on my 20D. Perhaps Canon is beginning to optimize their new lenses for digital cameras, because this lens is just fantastic.