Paul T Klenk: Yes, I, too, have a 24-105 L and it's a genuine workhorse. I use it all the time and get great results. So, why would I go to a 24-70mm f/4L? Well, only if the difference is spectacular and the additional macro feature is not going to do that for me. So, as the others have indicated, the actual images will be important. If one buys this lens through Amazon, the return policy is simply spectacular. Buy the lens, try it out, and if it doesn't meet your expectation, it can EASILY be returned. That may be my course to follow.
The new 24-70 f/2.8 II makes the original 24-70 f/2.8 look very soft and the colors and bokeh are phenomenal; this f/4 lens features some of the same optic enhancements as well as better hybrid IS and a macro feature. It will likely destroy the 24-105 in overall IQ - the 24-105 isn't really worth of L designation in most pros' opinions, it was a do-it-all kit lens for the 5D2/7D...but Canon isn't aiming at those owners; it's aiming at those buying a 6D.