I recently, a cpl mos ago, bought the 24-105 F4L IS USM,..I haven't shot a lot with it yet, but so far I like it. Last time I looked it was about $70 cheaper than when I bought it at B&H.
I read reviews on it and took the leap,...I wanted the IS because I can't seem to hold steady enough shooting off hand to get the sharpness I want. I use a 20D, so the crop factor gives me 38MM when set to 24MM, but I may upgrade to FF someday so......
According to Canon, the IS makes up 2 or 3 stops difference, so therefore I passed on the 24-70 F2.8.
My $.02
