I am sitting here comparing test shots made with this lens and Canon 100/2.8 macro, which, as we know, is a very sharp prime lens. This zoom is SHARPER in the center, has better contrast and color rendition. It is slightly softer in the corners wheres 100/2.8 is the same. The most amazing thing about this lens is IS. I used to have 28-135 IS and I was disappointed because with IS turned on it was significantly softer. But this one in all my tests even at 1/800 sec is as sharp or sharper with IS turned on than without it!
The build quality is not exactly the same as of 70-200 non-IS version because it does have a plastic barrel, but nevertheless it is very good. And it helps to reduce weight, which makes it an excellent travel lens. Highly recommended.