I've had a canon 28-105 3.5-4.5 for about 4 years on my 10D and was very happy. I have just switched to a 5D and have been playing with available-light shots where I wanted more than the 4.0/4.5 of the canon and still have wide-angle so I could use it as an all-round lens.
I was weery at first about sigma, but I was not willing to spend $1000 extra for the 24-70L or $400 more for a second-hand 24-70L. After getting home I setup the tripod and tested the 24-70EX to my 28-105 at 28/35/50/70mm and also my 17-40L at 28 and 35mm. Openings at 4.5/5.6/8/11...
The 24-70EX was very noticeably sharper at 4.5 than my 28-105 canon. No challenge. The 17-40L beat the 24-70EX in terms of sharpness and contrast, but the difference was not so great as with thr 28-105. This lens will stay in my bag for the next few years and the 28-105 will be sold off. A good step up for me.
Only downside is build quality. Although it is an "EX" it is still plasticy to feel. The zoom ring is very tight around 24-30mm and I always have to remove my hand from the lens when the AF is seeking, else if a finger is laying on the focus ring, the AF will not move anymore.