After working with EOS 1 Ds mark I up to markIII for 10 years I've now switched to the EOS 5 mark III.A few differences in handling I find very annoying.
-you can't see which exposure mode you're using in the viewfinder!
-in A.P,S mode,when you lock exposure,you can't see how much you're under/over exposing in your new composition.
-and when switching from A to M mode for example, you'e aperture switches to the last one you used in M instead of the last one you used in A mode.This means that when switching between exposure modes in a given situation you have to adjust more then you would like.
-I also don't really like that the C1,C2,C3 function doesn't remember the changes you make during shooting (e.g..changing ISO) when the camera falls asleep.
My best handling camera ever was the EOS1D markII , best camera the 5 mark III (of course)