I have had a 24-105 f/4L for five or so years now, using it exclusively on APS-C bodies. Currently, I shoot with a 70D.
Of my regular-use lenses, the 24-105 is one of the least impressive. It is a reasonable all-around lens, but the sharpness and overall look trails other lenses I use regularly, such as Canon's 10-22, 70-200 f/2.8L IS II and the times when my Sigma 18-35mm f/1.8 throws a sharp image. The 24-105 produces consistent results, has good build quality and is quite reliable. However, the images just don't "pop" in the way some of my other lenses do.
For my money, the 17-55 f/2.8 is a great all-around lens. I've rented two and used a third copy that a coworker has, and for IQ, speed, AF and all-around usefulness, it is my first choice on crop-sensor cameras. I will hopefully purchase one in the coming year. For me, it's main downfall is the build quality. I often shoot outdoors in dirty/dusty conditions, and the 17-55 (and reports of internal dust building with lens age) doesn't inspire confidence. However, I will use it mostly as an indoor lens and outdoors in non-dusty conditions.
I have the 17-55 and have not had any dust issues.