I was a bit hesitant about getting this lens for my 5D rather than the 24-70L having seen one or two rather negative reviews here. In the end I got a good deal on the lens from Jessops and have not been disappointed. I can't understand folks who buy a 24-105 then say they wish it was 150! You bought it knowing what it was so it's your fault, not the lens's.
Overall I was delighted with the performance of the lens.
Sharpness wide open at 24mm not quite up to my 70-200L f4 but, hey, you expect that for a wide optic zoom and it was still acceptably sharp & retained it's contrast at f4.
At 70mm and 100mm you can't tell the difference between the 2 lenses at any aperture, even on A3 prints. IS means more keepers, though, and I regularly shoot 1/25th or thereabouts at 105mm (which I couldn't do with the 70-200L where I'd be looking at at least 1/100th. or faster)
As a very subjective test I shot a scene at 50mm, f5.6 and ISO 100, and the same scene with Contax 35mm (Zeiss Planar 50mm 1.8) on ISO 100 film, again at f5.6. I had the film professionally scanned at high resolution (no, by a pro lab - not by a one-hour high street photo outlet!) and printed both at A4 (Epson R800 and Qimage) - absolutely no difference in sharpness, detail, contrast or smoothness of image that I (or any of my friends) could see. Some preferred the Zeiss/film colours, others the digital but I could have made them identical in DPP, DxO or PS had it been important. The bokeh was better with the Zeiss but I'm splitting hairs.
I have since revisited locations I shot in B&W with a Rollei 6004 (80mm Planar) and tried to replicate with 5D and 24-105 at 80mm.
It's pretty darn close but there's a subjective "niceness" about the Rollei images that are nothing to do with sharpness or detail.
I'm very happy with the lens but might get a 50mm 1.4 for carry-around.