For shooting candids at events or sightseeing, I often carry a 7D/70-200 2.8L II and a 60D/17-55 f2.8. But, there I times when I wish to travel a bit lighter and swap out the 17-55 for a 35 f2.0 -- or carry one body and the 35. I know the 35 is soft in the corners on a full frame body, but it's quite sharp for my crop bodies.
I must admit, it is hard to do without the 17-55 when I need to cover an event. But, when I'm shooting only for me, if I'm going to limit myself to a prime, I like the 30-40 range for grab shots.
Now, I'm intrigued by the pancake 40. It's half the size of my 35, but one stop slower. But, I'm having trouble justifying the loss of a stop for the savings of about 0.8" in camera/lens depth. On a 60D or a 7D, does the pancake make that much of a difference over the 35 in the convenience department?