I went from a Tamron 17-50 F/2.8 VC to a canon 15-85 is usm and it is miles better, very sharp, silent AF, vibrant colours. But before I got the 15-85 I was seriously considering the 24-105 as it is a stunning lens and I was advices by many to go for the 15-85 instead as its a more usable lens on a crop body, it's the equivalent of 24-136 of full frame and some of the people that adviced me on Flickr went from 24-105 to 15-85 and they said they were very happy with they'r decision although I would probably still get the Tamron 24-70 F/2.8 VC USD too (once the price drops down from £899) as it is a stunning piece of glass.