I used to have a Pentax 40mm pancake lens that I bought to use with my old Pentax ME Super. That was a fairly small 35mm camera, about the size of a modern Canon Rebel without the grip. I made my own very thin case so that I could carry the camera in a coat pocket. I thought that was so cool!
Unfortunately, shortly after that, I discovered a small zoom lens that was pretty sharp and that quickly became my favorite lens for the ME Super. The pancake lens stayed on the shelf for many years before I sold all my film camera gear. I got a good price for it on Ebay.
I am drawn to the new Canon pancake lens for the coolness factor, but realistically, I can't see a use for it in my system. Putting it on one of my full sized bodies seems pointless, since I would still have a large camera to lug around.
I could put it on my T2i, but the 18-50 kit lens is really light, sharp and has IS. I've been very happy using this combo as my vacation camera. I'd miss the zoom capability.
I have an S90 that fits in my pocket for times when I need that.
I just can't think of any use for a pancake lens other than to look cool in the cabinet where I keep my lens collection.