I have the 24-105 with the 6D and I find that it's a good General Purpose lens. That being said I tried the Tamron for a couple of weeks and I didn't care for it, particularly when compared to 24-105.
The issues I have with the lens is that (as noted) IQ is fair. It does get better from about F8 on, also it has a tendency to overexpose. Others have had this issue, it appears that the lens mis-reports the aperture setting to the camera.
The compromises are more acceptable if you can find a good used copy, but as a new lens I passed on it. I recently got a deal on a used copy for about $200 less that new, at that price I can work around the flaws