I picked up the 40mm the last day it was $150 at Best Buy, figuring that I could always return it if I found no use for it. I only finally got around to using it a few days ago, the return period already behind me. Fortunately, I'm thrilled with it:
1/4000s | f/9 | ISO400 | 40mm
1/125s | f11 | ISO500 | 40mm
My shoulder bag gets pretty hefty with the 70-200 f4L, 100mm f2.8L, and 24-105mm. The 40mm just replaced the 24-105mm, and I stitched panoramas out of 40mm shots when I wanted really wide shots (having moved to full frame really makes this more viable):
1/40s | f2.8 | ISO800 | 40mm (six or seven shots merged)
I'm thinking that's a lot of performance for $150 and 4.6oz...