So I have a 15-85 IS USM on my 50D and I'm very happy with it. In fact it's now my bench mark that I measure my future lenses. Basically I want to buy some other lenses that will give me IQ close or better than my 15-85. So far my lens short list...
1.Canon 10-18 or 10-22 for ultra wide landscapes
2.Canon 70 - 300 IS USM
2.Canon EF DO IS 70-300mm
2.Sigma 150-500 IS for taking pics of the moon and close ups like wild life
3.Canon 100L Macro
For my Son who will get my 40D, at the moment with no lens
1.Canon 18-55 IS
2.Canon 18-135 IS
It will take some time to get a selection of gear but I post this up for advise on which lens. I like sharp pictures. The Canon 17-85 IS USM is no good for me, never impressed with IQ.