I used to have the original 24-70L, sold it and pretty much got the Tamron for no cost. Much sharper and image stabilization without having to shell out any more cash. New canon might be sharper than what I have but I think the tamron was good value for the money for me.
I also replaced my canon 100-400L with a Sigma 120-300 f2.8 (and 2X teleconverter), and likely will replace the canon 50 1.4 with the new sigma (already have the sigma 35 f1.4). I hadn't really look at it but I seem to have been replacing a lot of my canon lenses lately.
Next question would be weather to get the new sigma 24-105 to replace the canon 24-105L?
I went the S24-105 route and couldn't be happier