There's not really a question here, but I wanted share a few observations about this lens. I just purchased it last week and I have used it thus far for a glamour shoot with a new model and for photographing a large festival. I have the option of returning it within 30 days, but after these two shoots I'm definitely keeping the lens.
Here's what I like about it:
1. It's much lighter than my 28-70 f2.8L and although I haven't done any critical testing, it seems to be as sharp as, if not sharper than that lens as well.
2. The 28-70 always left me feeling short of reach at fairs/festivals, but using the 70-200 f2.8L was just too large and heavy of a lens to carry for hours on end, plus I couldn't go wide at all. The 24-105 focal length was perfect for the crowds, the festive groups of people, and for reaching out a isolating an individual. I've always liked that focal range, but I never cared for the quality of my 28-105 USM lens when compared to my L lenses.
3. The lens quite sharp, even wide open. The IS is almost silent, and there's very little vibration. It's sometimes tough to tell that it has kicked in. But, I can tell that it makes a significant difference.
4. The color quality of the lens is consistent with my other L's, and the bokeh at f4 is better than I expected.
What I didn't like:
Actually, I can't think of anything right now.
If this lens hadn't performed to meet my needs/expectations I was fully prepared to return it for a refund. I had read quite a lot of good reviews about the lens on this forum and others, and I'm pleased to say that the lens lives up to its well deserved reputation.
If anyone is interested, I'll post a few shots from the fair later.