I've said good things about the Tamron 28-75 before, since it was the only lens that would really focus well on my persnickety old 5D Classic.
I've had my 5D3 for about a year. About six months ago I figured the 5D3 deserved an L zoom so I picked up a 24-105, F/4. It's a great lens, very sharp, with fast autofocus and effective IS. My old Tamron sat on the shelf.
Recently I was doing a long studio shoot and started to feel the size and weight of the 24-105. For the next shoot I decided to try the much smaller and lighter Tamron. I was surprised to see how well it worked.
The autofocus isn't quite as fast as the 24-105 and it does make a little bit of noise, but not enough to be noticeable in the middle of a photoshoot. The important thing as that the images are very sharp. Of course, I'm shooting with strobes that prevent motion blur and I'm using enough light to permit apertures around F 5.6, so even a cheap lens will create sharp photos. Under these conditions, at least for me, an L lens is overkill.
Since the Tamron is considerably smaller and lighter than the Canon, I am a happier photographer during a studio shoot. I will still use the stabilized 24-105 for outdoor shoots, but in the studio, I'm back to the old Tamron, which, by the way, I picked up on Craigslist for $260 about three years ago.
This photo was at F/3.5, 53mm, ISO 100 (Alien Bees with umbrellas). I wanted to see if the Tamron would substitute for my 50mm Sigma in shallow DOF portrait situations. I think I actually dialed back the sharpening in Lightroom.