I used to own the 28-70L, i've shot with a good copy of the 24-70v1 and i've bought and returned 3 copies of the new 24-70mk2 and tried 2 others in-store, plus borrowed one from a canon rep...
I can see the potential in the new version, it's disappointing i haven't found an ideal copy yet but i've seen really sharp results in places, really good contrast and colours and very little distortion, focusing is fast but not as noticeable over the 28-70 that i had as i'd expected.
The problems i've found are mainly soft sides at f8 and one that wasn't sharp at close focus distances (halination??).
I do see the differences and will buy a mk2 version but i don't think it's $1000 better than the 1st version... it is $1500 better than the 28-70L though, much more usable wide open and the extra 4mm.