Before buying the M + 22mm (and being very impressed by how accurate its focus is when you use the zoom thingy on the monitor for small objects) I thought I would never want a camera without a viewfinder and a fixed focal length (I realize it's not fixed on the M, but for me it is for now), especially one that's relatively wide. But as long as they're fairly inexpensive relative to the price of a lens by itself, it occurs to me - as it has to others - that they're a good alternative to carrying around another lens: if I have to carry another lens around, I might as well carry another camera if it's small enough and save myself the trouble of changing lenses. (If this M + 22 deal had been available when I bought the Olympus 17mm f/1.8 for my OM-D, I would have bought the M + 22 instead and saved myself $200!)
Two or three would probably suit me in addition to the M + 22 - 50mm equiv. & 70mm equiv, and maybe 24mm equiv for occasional use (more if a small longer lens were feasible). If just one, I guess 50mm equiv.