Well, now things are more complicated for me. I own the tammy 17-50 VC for my crop body canon. The image quality is good enough for me, but focusing is slow and noisy.
Lately I have been renting different lenses from a local place here in Houston. The more L lenses I rent, the more impressed I am with the build quality. There really is a huge difference. My 17-50 VC feels like a toy compared to the 24-105 f/4 L. The canon lens just feels way more significant. And the canon USM focus speed is blindingly fast.
So i was trying to choose between the 24-70 L and the 24-105 L. Rented both several times and tried them in various situations. Love the 105mm reach of the 24-105 L, and it also has great IS. But I also love to melt away the backgrounds with the f/2.8 of the 24-70 L. Both are built like a tank and really inspire confidence. It is a tough choice but I was leaning towards the 24-105 L because it is slightly cheaper and I like the extra reach and IS.
Now the new Tamron 24-70 f/2.8 IS has been announced. It solves two out of 3 issues for me (if it proves to have good IQ). It has IS, and it will likely be priced below the 24-105 L. It doesn't have the 105mm, which I absolutely love, but 70mm is getting into pretty decent portrait territory for the way I use lenses. I have heard good things about Tamrons USD system. If it turns out to be anywhere near as good as canon's USM, I would be happy with it. So then all that remains is build quality and that feeling of significance. From what I have seen canon has the market cornered in the aspect. I don't mind carrying the extra weight, it doesn't bother me.
If it is priced in the 700-800 dollar range, I think I would take a chance. Because I could get this and a 100 f/2 and be a pretty happy camper.