I like the 24-70mm f/4 IS very much. I originally bought the f/2.8 MkII, it is a fantastic lens, however I need the IS for low light event shooting. But, back to the subject at hand, while I had the 2.8 MkII I rented the Tamron, in my opinion, not as good as Canon. When the f/4 came out, I rented it and compared it to the f/2.8 and "for what I need" the f/4 was the winner and I returned the f/2.8 and bought the f/4. All comparisons done on a 5dMkIII.
Price was not the issue, the new hybrid IS on the f/4 allows me more than enough room to make up for the one stop difference and I get all the benefits of IS (yes, I give up one stop of DOF, but check out the DOF calculator - it is minimal). Where in the world these folks are coming from saying the 24-105 IQ is better than the 24-70 f/4, either don't own the 24-70 f/4 or they are trolls. The IS on the 24-105 is old 2nd generation and does not hold a candle to the new 24-70 f/4 and 70-200 f/2.8 MkII. This hybrid IS is rock solid.
I just wish the naysayers would come out and honestly say whether they have actually shot with the lens or not. Further, on an actual shoot and not shooting a bunch of test circles. (I have never been paid a penny for test shots). Finally, I need IS and it is my money, so don't critisize me for my shortcomings (unless you are willing to pay good money for it).
I love Canon products and applaud them for offering a wide range of great products with a wide range of price points. Finally, if they introduce a f/2.8 IS, I would strongly consider buying it just because I can!