I do my serious shooting with a 5D III, but I had been looking for a while for a small, lightweight camera for family pics and travel. I had considered a high end compact but it somehow seemed too much of a compromise, particularly in operation (shutter lag, viewfinder, etc). When the SL1 came out it seemed an ideal solution, and after having spent some time with it I can say that for me this camera (with the 40mm pancake) is almost perfect as a lightweight complement to the 5D/III. The great thing, for me, is that both cameras share so many things - the optical view finder (obviously), the Canon layout of the buttons, and the general shooting method. The only real difference in operation is the lack of back button focus (and the lack of the second wheel).
I feel Canon produced a wonderful little camera in the SL1, and for me it is the perfect "informal" complement to the amazing 5D III. I figured others might be wondering about this so I thought I'd share my positive experience!
I still think the missing piece for the SL1 is the wide angle pancake. The pocketability of that camera lives or dies with a tiny lens (as it does the EOS-M), so pancakes are vital. But the EF 40mm pancake scales to 64mm FF equivalent on a crop, which is functional for portraits but too long for general walkaround, IMHO.
So the missing piece is what they do offer for the EF-M mount but not for the EF-S mount: a 22mm pancake. That scales to 35mm, and then you have a truly high-end compact 2nd camera.