The most fun lens I own. Unbelievably sharp right out of the box--no need for micro-adjustment. Excellent color, too. Of my four IS lens, this one is the most effective.
I was simply blown away by the image quality--I'll even give it the nod over my much loved 70-200 f4L IS. If you can justify the expense, run--don't walk--and get one. BTW, I bought mine during the Canon spring sale, so that is why it's a little cheaper than normal.