Yeah, my impression is that the 24-105mm is a great lens, based on what I’ve read. But again: the copy I had didn’t convince me. It possibly could’ve benefited from MFA, but after I was aware of the problem I did some tests with manual focus, aided by LiveView, and it was still pretty soft wide open. I really want to try a new copy now, to see if I was just a bit unlucky. After all, the build quality was excellent and I do miss the weather sealing.
Regarding the Tamron 28-75mm, you are depending on getting a good copy. I did, and was also able to buy a wider prime for landscape-photography for more or less the money I saved/made from the purchase and selling of the 24-105mm. I would definitely try the Tamron out before I buy it. They do make some great lenses, but are often overlooked because of a lower re-sale value, inconsistent quality control and not-as-great build quality. I’ve read online about photographers doing blind-tests with the Tamron 28-75mm and the Canon EF 24-70mm, without being able to find a clear winner. These specific photographers skill levels are unknown, however.