Often compared with the 24-105 and I guess purely from the numbers, zoom range and aperture.
My copy of the 24-105 had pretty bad CA which was especially noticeable when shooting into sunsets and the corner quality was really shown up on my full frame.
The the question for me was really to go for the 24-70 2.8, 4 canons or to go for the Tamron.
The Tamron I tested in store had major C.A which left the two canons.
I went for the f4 mainly because I don't need the 2.8 on a standard zoom and because I already have the 50L I thought that the would settle for the zoom rather than switching over.
The f4 is close to the perfect all round lens, weight & size, zoom range, IS and for those who want a part time Macro lens.
Images are sharp the auto focus is quick, though captures these have mainly been landscapes and I have yet to test it out on moving subjects or people.
This is is definitely coming with me on my 6 week trip to India, Nepal and Tibet later this year.