I currently own a Canon 60D, and I am perfectly happy with the camera. It is going to be years until I move to a better (most likely full-frame) body.
My main, walk-around lens is the 15-85. I am also very happy with this lens, although I also know its shortcomings, like the variable aperture (3.5-5.6).
But this morning, as a "what if" exercise, I started thinking about upgrading this lens.
Since my 60D is a crop sensor body, the closest and cheapest upgrade I can do is the 17-55 EF-S USM IS. It has a constant aperture of 2.8, and people say that it produces the best picture quality of all EF-S lenses. The upgrade would probably cost me $300-$500, depending on the price I sell my 15-85 at, and whether I want a used or new 17-55.
But 'upgrading' from an EF-S lens to another EF-S lens is not really upgrading, is it? It is more like switching between two similar quality products. Is it worth it?
A TRUE upgrade would be to get an L lens. So, assuming that price is no object (one can only dream :(), what L lens would you recommend to replace my 15-85?
EDIT: What I would look for in the ideal replacement lens.
Thanks for all the answers so far. One conclusion I came to after reading them is that there is no point for me to invest in L glass if I care about wide angle on my crop sensor body. And I do.
I like my 15-85 because it offers me the flexibility of being able to capture moments like a few snapshots in a restaurant with friends, or my 2 years old daughter who won't sit in one place for long - definitely not long enough for me to switch lenses.
I could give up the 2mm difference on the wide end of the spectrum, if the 17-55 offered me much higher IQ than the 15-85 does.
Hence the followup question - Please let me know if you upgraded from 15-85 to 17-55 and if that was a right decision.. I know it is all personal - and that is what I am asking about - what was your experience with such upgrade if you did it?