Just wanted to mention it: There are other excellent lenses out there that create stunning images except for Canon "L", and older Canon L or even the current 17-40 aren't that stellar. I've got Canon lenses that don't have a comparable 3rd party alternative (100L, 70-300L) (...)
In all fairness, the Sigma 105mm f/2.8 OS HSM is comparable to the 100 L, and on par mechanically and IQ-wise. It has a metal body, the 100 L is a bit on the plasticky side in my opinion. The 100 f/2.8 non-L is also at least on par IQ-wise (except the L's bokeh, which is great).