I’ve had time to play with a couple of copies of this now, shoot it a bit more, and make a firmer opinion. It is a bit sharper throughout the range than I first realized and is a clear improvement over the consumer zoom at the long end, but then at three times the price, it should be sharper. I stick with my opinion that it’s not Canon 70-200mm f2.8 IS II sharp but certainly Canon 100-400mm sharp. There is no major distortion evident at either end.

It is not as large as I expected- clearly lighter and smaller than a 70-200 f2.8. The front element extends on an inner barrel about 3" when zooming, but the front element doesn’t rotate, so filters will work fine. The IS is excellent. One spectacularly good thing I found only after working with it for some time: it may be the most flare resistant lens in the bag, so its an excellent choice if you’ll be working with bright sunlight in the field of view.

Still, I’m sticking with my original opinion. It’s a very nice lens. It’s a much better lens than the $500 consumer grade Canon 70-300 IS (which is a very good consumer zoom). However, if I were putting down my hard-earned cash (OK, maybe not so hard-earned, I make my living playing with photography toys all day), I’d buy the consumer zoom and spend the other $1,000 on a Canon 17-55mm IS or Canon 24-105mm – or just get a Canon 70-200mm f/4 IS and blow $500 on groceries or something frivolous.
April, 2013