I have both the 17-55 f/2.8 and the 17-85 f/4-5.6 (the latter came as a kit lens). Mostly I use the 17-55 as I shoot a lot in low light situations (e.g. church interiors) and it's OK for landscapes and walk-arounds too. The 17-85 actually has a much nicer focal range for walk-arounds. Although it can't match the 17-55 at the widest angles (it suffers badly from colour fringing and softness here) it is pin-sharp at the longer ranges - far superior in this respect to the 17-55. The main thing I don't like about 17-85 is that the colours appear 'flat'. For all its sharpness the images are a bit "meh" - which inevitably means that it tends to get left in the bag.
I have been wondering for some time whether to ditch the 17-85 for the 15-85. In theory it could replace both the above lenses. However I don’t think I could ever give up the constant f/2.8 of the 17-55 and I have read elsewhere on this forum that this lens is sharper than the 15-85. If the 15-85 isn't as sharp as the 17-55, then it sure as hell isn't going to match the sharpness of my 17-85 at the longer ranges. So I suppose it boils down to the IQ/colour/contrast handling in the 56-85mm range. This seems a fairly narrow band on which to be spending what is for me quite a lot of dosh. Would I really be gaining that much in IQ if I got a 15-85?
I have looked through the threads for each lens in the Lens Sample Archive, but even the great pics there haven't really helped.