Bill Caulfeild-Browne has written a good article comparing the new Canon 24-105mm f/4L IS USM with the legendary Canon 24-70mm f/2.8L with Canon 1Ds Mark II @ Luminous Landscape. This is what he says about:
Distortion: Both lenses show significant barrel distortion at 24mm, and pincushion distortion by 50mm. However, Canon 24-70mm shows less distortion than Canon 24-105mm.
Vignetting: Canon 24-105mm has quite pronounced vignetting at 24mm and it’s still apparent even at f/8. While the Canon 24-70mm shows little vignetting even with f/4 or smaller
Resolution: Both lenses are essentially indentical in their resolution, but the winner goes to Canon 24-105 for showing higher contrast than the Canon 24-70
Macro: Canon 24-70mm will give you 0.29 magnification, while the Canon 24-105mm gets you 0.23. The IS in Canon 24-105mm is really usefull when working close up.
Action: If you intend to use the lens to freeze actions, then you’ll need the extra stop from the Canon 24-70mm. On the other hand, the third generation IS will give you extra 3 stops when shooting in low light.
In the article Michael Reichmann added:
“Though I have just started to use the new 24-105mm, and unlike Bill have not done any rigorous testing, the frames that I have taken have been very impressive, and the lens’ handling has been exemplary. As this is being written I leave in a week’s time for a major workshop / shoot which I’ll be conducting in China during the second two weeks of October, and the 24-105 has definitely taken its place in my bag, replacing the 24-70 as my general purpose medium focal length zoom.”