Using only unmodified raw files from the 20D I examined the results from 5 focal lengths (83mm, 50mm, 35mm, 24mm, 17mm) at 5 f-stops (f/5.6, f/8, f/11, f/16, f/22) in Photoshop CS. I shot the test in late afternoon daylight, no flash. I used ISO values ranging from 100 to 1600 as the aperture was reduced so that the shutter speed stayed above 1/5 sec. I was shooting off a good Manfroto tripod with a good Gitzo head. All frames were taken using the manual mode.
I had to turn off IS because autofocus would hunt continually at 85mm and 17mm. With IS off autofocus worked perfectly. I have had this problem with this lens from day one, handheld or on a tripod. IS/Autofocus seems to work fine together at the in-between focal lengths.
At the short focal lengths there is some color fringing, especially noticeable at 17mm in the corners and along the short sides. At 50mm and above there was no noticeable fringing and the in-between focal lengths were proportional between these two.
At 17mm there was a lot of softness at the edges when the center was in focus, even at smaller apertures. At max aperture for 17mm the whole image was quite soft but it sharpened up in the center by f/8.
All photos taken at 17mm show up as a 56mm focal length in the Metadata, other focal lengths are reported correctly.
I know that there is a natural problem that occurs as the aperture gets smaller than about f/22 (diffraction limit) but it seems to me that all of my lenses get a little soft (even the expensive L lenses) once the aperture goes below f/11 or f/16. This lens is no different, the images tend to look a bit softer at f/16 and smaller than they do at f/8... but this is nit-picking by comparison to the color finging and softness at 17mm.
Because of my experience with this lens I have only purchased prime lenses to fill out the needs I have encountered.