Good grief, you guys have sort of put me off the 24-70. I've been contemplating it or the 24-105.
I already have the Tamron 28-75. Apparently I wouldn't gain much image quality, eh?
You can do what I did - buy one used, use it for a month and sell it if you don't like it. But don't go for the cheapest one, they are cheap for a reason and you may not be able to turely experience a good copy of that lens. I tried 3 copies.