I had a 600D / 17-55 and upgraded to a 5D3 / 24-105 and 24-70 II.
The 24-70 II blows the 17-55 away.
- It is much sharper. f/2.8 is outrageously sharp
- I find the colours much more appealing. The saturation is much higher, almost so much that I have to desaturate photos sometimes. On the 17-55 I was usually adding vibrance +10 to 20 and saturation +5 to 10.
- Extra (ie reduced) DOF (about 1 and 1/3 stops) is great
- I find the bokeh more pleasing
I find the 24-105 to be better than the 17-55, but the 24-70 kills both of them hands down.
I do with the 24-70 II had IS, but for capturing social events etc I can still shoot at ISO 25,600 and f/2.8 and produce OK (but not great photos).
ISO 12,800 and f/2.8 is fine so long as the exposure is right.