I just placed an order for the 24-105L. It will be paired with the 80D. Hopefully I get similar results to what some have gotten here. It is surprising there isn't very much activity in this thread.
My 28-300L is great and performs well bare, with filters and stopped down but the tension ring doesn't work anymore. Zoom creep on it is more like slide/thud! Yes, sending it in for repair is an option but that could cost half of what the 24-105L costs. So it got me thinking that around 80% of my landscapes are within the 24-105mm range. The 100-400L could take care of the rest of the long end. It is possible the 28-300L would feel unloved.
I have the 18-135 but few, if any, of the images have ever made it out of LightRoom. It appears the 24-105L is smaller, half the weight and at least as good as the 28-300L but with the added benefit of 24-28mm (something that I've wanted several times). Hopefully it likes filters and being stopped down as much as the 28-300L.
The 15-85 was also a consideration because of its sharpness but zoom creep seems to be common and it isn't quite long enough.
Hope to be posting soon.