I just got the 24-70 2.8 II last week. Ready to sell the 24-105.
Reason I replaced it: For me, on my 5DIII, only works under perfect conditions of light and distance. On cloudy days, unpleasant contrast issues, similar to Clarity slider being too far right. Also, faces at medium distance simply don't resolve well if I want to crop, so forget about further back. Distortion, making group shots always problematic. Occasionally mutinous IS.
Only reasons I consider keeping it: 1) Wife is sometimes a second shooter, and she, I hate to say, has butterfingers.
2) If I go into crime ridden areas, I'll bring it and my 60D.