Same here! I just received a refurbished 15-85 from Canon. Testing it on my 60D, I have also noticed very inconsistent results in terms of sharpness. When it's sharp, it's SHARP! But 30-50% of my photos are coming out noticeably "soft". It's especially obvious when I take several consecutive photos of the same subject, and half of the photos are clearly sharper than the other half (with the same camera settings, etc.).
My current assumption is that in my case it's probably due to user error with the IS. It seems to me that the IS on this lens takes a few seconds longer to "kick in" compared to my other IS lenses (17-85, 18-200). For example, I've noticed that I get much more consistent sharpness if I wait 3-5 seconds after tapping the shutter release and hearing the IS activate before I take the picture. It's almost like the IS takes a few seconds to stabilize, unlike my other lenses that seem to stabilize instantaneously. I understand the IS in the 15-85 is supposed to be a more sophisticated implementation that what is in the 17-85 and 18-200, but maybe "better" doesn't mean "faster"? This would be a real bummer for me so again I'm hoping it's user error as I get used to this "new" version of IS.
Another reason I suspect the IS because from a tripod - with IS off - IQ is great. Easily and dramatically sharper than my 17-85 and 18-200 at the wide end. Though surprisingly, my ancient 17-85 is a bit sharper at 85mm. My 60mm EF-S macro is noticeably sharper at 60mm, but this doesn't surprise me since the 60 macro is the sharpest lens I've ever owned (Canon or Nikon). So under controlled conditions, I'd say this 15-85 is as good as I was hoping for. But under more dynamic shooting conditions, I'm a little bummed that so far my older and "lower class" lenses are more reliable!
Anyway, I hope you decide that your 15-85 is OK. I too have read overwhelmingly positive things about this lens as well as the quality of Canon refurbished products in general. Maybe both of our expectations are therefore unrealistically high (it is after all a consumer zoom). But I remain optimistic that in my case I just need more practice with it. The range is ideal, the build quality is nice and the sale price from Canon Refurbished was great. But if you come up with any major discoveries with yours one way or the other, please let us know!
David