Here's my opinion based on experience with 17-55 2.8 IS on close to a dozen crop bodies, plus the 24-105L and the Tamron 24-70 VC both on crop and FF bodies.
I had 2 copies of the 17-55 back then. First one was sharper than my second one but the IS motor crapped out, got it fixed, however, after that this lens mysteriously did not AF correctly on my first 7D body with the center AF point. So 1 specific lens on 1 specific body with 1 specific AF point. No problems on other bodies, and the 7D had no problems with other lenses, it was weird.
Anyway, my second 17-55 was a little less sharp but still descent (first one was super sharp). I traded it for a Sigma 17-50 2.8 OS which also was a good lens.
Then I moved full frame but I still have my 7D and my fiancee has a T4i. I sold the Sigma 17-50 because I bought a 6D kit with the 24-105L. Then not long ago I also picked up a Tamron 24-70 2.8 VC.
Ok, after this long intro, here are my thoughts:
- My 24-105 is just as sharp as my 17-55 was, the first one that is. It works great on my 6D, great focal length, IS works well, however, sometimes the extreme corners are less sharp (full frame). It is awesome on my 7D too, no softish corners there, since it's a crop.
So I wouldn't consider it a step down after my 17-55 IS.
What's also notable is that the depth of field with the 6D + 24-105L is similar or maybe even a tiny bit shallower than the 7D + 17-55 IS (I think DoF of the 6D +24-105L is somewhat similar to 7D plus a f/2.5 lens.) Also, with the 6D's excellent high ISO performance f/4 + higher ISO is actually just as usable on the 6D, or even better IQ, than f/2.8 was on the 7D (7D + 17-55 2.8 IS) at shooting low light events.
The Tamron 24-70 2.8 VC is also a very good lens, I'd say close to the 17-55 2.8 IS but in a FF version. It's obviously not as good as a 24-70 II but fairly close (how close depends on copy variation I guess).
The 24-105L and 24-70 Tamron spends about equal time on my 6D. They are both good lenses, but if budget is tight you don't have to have both. It depends, the 24-105L is a very good general use lens, 105mm is nice to have and it is fairly sharp at f/4 using it wide open is a breeze.
But if someone shoots a lot of indoor, low light events, the Tamron would be the better choice. I shoot events fairly often, and planning on shooting a couple weddings too in future so I do need the 24-70.
I have no experience with the new Canon 24-70 f/4 IS, but if I was in the market for a lens for my 6D, I personally would not even consider it due to price, focal length and aperture. For me the Tamron or the 24-105L would be higher on my list. I feel a little bit like Canon answered a question that was never asked by bringing out the 24-70 f/4.