I'm a long time prime advocate but took the plunge and bought this lens. I basically justified it by thinking, Iwould keep it only if it compared favorably to my 50mm 1.4 and 85mm 1.8.
I shot about 60 different shots at different apertures from a tripod (obviously, turning the IS off on the 24-105) at 50mm and 85mm, as well as at both ends. At 50mm and 85mm this lens proved as sharp as both primes at f/4 and even sharper at 5.6-11. I was amazed. There were very slight differences in color saturation when I printed the images, but I had to work very, very hard to see them. When I did a blind test, I was completely unable to distinguish which prints came from the primes and which came from the zoom except in those instances where the zoom proved a bit sharper at the center of the image.
I've since shot about 300 photos, including my son's basketball game in adverse light conditions. The IS worked extremely well. The renderings are very pleasing. No disappointments. So for those who are like I was, a doubter when it comes to using other than prime lenses, I must admit this proved to be a very pleasant surprise and learning experience.
Of course, it's not cheap, but I ended up trading in three prime lenses to offset some of the expenditure.