If this lens had been around a few years ago, I probably would not have dropped Canon. That's how important the combination of zoom range and image quality are to me. I went back to Nikon both because we already had a houseful of their lenses, and specifically because of their 24-120VR.
If I can "interpollate" a bit between Nikon and Canon, here's my experience. I'm really happy with the lens for "walk around" shooting, and especially in dusty conditions. The less often you have to change lenses, the better off you will be.
Though the 24-120 isn't absolutely critically as sharp as either my 20-35mm f/2.8 or my 35-70mm f/2.8, I already owned those and I use them when critical sharpness is required- especially landscape. In fact, that was a large part of my decision to abandon Canon- I already owned those lenses in the Nikon line, but was faced with buying similarly sharp lenses to go with the 28-135 if I persisted with Canon.
My summary of the situation, based upon my own shooting needs is this: If the 24-105 is as sharp as the 24-70, you are ahead of the game. If it's not, you may end up owning both lenses if the zoom range is critical for dust or diversity, yet the 28-135 doesn't measure up.
