Can somebody explain the appeal to me? Not trying to start a flame war, just trying to understand.
A 24mm pancake (which won't have IS) seems a bit redundant with the 24mm 2.8 IS, which by all accounts is a very good lens, has USM and which Canon cut the price to a much more reasonable level. The size seems kind of irrelevant once you put it on a 5D, 6D or other full frame body and for an SL1, you end up with a 37mm lens which is barely in the wide-angle realm.
Do people like these pancakes just because they are cute (no argument there)? What am I missing?
Size size size. It makes your rig so small you don't mind leaving the bag behind and just slinging your camera around your neck all day. Or it's such a small item that it's a no brainer to throw it in your bag as another FL option.
There's also a side argument (that some would refute) that the smaller your entire rig is, the more likely you'll bring it at all to take pictures.
But it will never 'compete' head to head at a feature level with larger lenses that offer IS, USM, weather-sealing, mechanical manual focusing, etc.
So I see pancakes as a nice option to reduce size when you don't need all those features -- leisure, walkaround, and street come to mind for lenses like these.