As noted, there have been a few copies with dust problems. Also as noted, the dust doesn't seem to affect image quality.
I've looked carefully at my two Canon zoom lenses (24/105L and 17/55). The quality of the build is not the same; the 24/105 is obviously better built.
As for the dust situation, the major difference in the two lenses can be seen when one attempts to slip a thin piece of cardboard between the zoom ring and the barrel. On the 24/105, the cardboard stops very quickly against what feels to me like an O-ring, whereas on the 17/55, there is no seal, and the cardboard slides in easily.
A simple test was done by pushing a 1/4" (6mm) wide strip of light cardboard (really heavy paper) between the zoom ring and the barrel: On the 17/55 I was able to push it 1 3/4" (45 mm) into the lens. On the 24/105, the strip of cardboard won't even go in 1/8 of an inch (2 mm).
The most significant difference in the two lenses is that the position of the zoom and focus rings is reversed, meaning that going from one lens to the other is confusing.
The image quality of the 17/55 is at least as good as the 24/105, and the colour to me seems identical.
I have no dust whatsoever in the 17/55, but I'm one of the idiots that puts a UV filter on every lens. However, considering the lack of a seal between the zoom ring and the barrel, I highly suspect that a major point of entry for dust is at this point. Having always been aware of the potential dust problem, I always wipe the exterior of the lens with my fingers or a cloth to remove any dust that has collected on the barrel at the juncture with the zoom ring as I feel it could be sucked in when the lens is zoomed out (the increase in length results in an increase in volume which can only be made up by incoming air).
IMO, this is a killer lens. Photozone tends to agree:
Look at the MTF charts for this lens compared to other lenses; very few lenses even come close to, and even fewer exceed, the IQ of this lens. In the centre, it tests better than my 100 mm f/2.8 macro.
One other comment: I use this lens for indoor people shots (birthday parties, etc.). With the fast f/2.8 and the IS, I don't use flash; just set the ISO to 1600, and in a reasonably lit room, flash isn't necessary. One can get a lot of good candid shots with this lens.