As stated, if you do the basic geometry, 43mm is the closest focal length to a true "normal" as determined by the diagonal of the 24mm by 36mm frame. A lot of fixed lens rangefinders (Rollei, Yashica Electro 35, Minolta Hi-Matic, and others, used 40, 42, and 45mm lenses and I suspect when the CL was developed, given its size, it was meant in part to compete in the market for those very small fixed lens rangefinders, with the added benefit of interchangeable lenses. Anyway Erwin Puts in the Leica Lens Compendium writes about Leica's early 35mm lens development, when they were the only game in town, and on the choice of 50mm.
I liked my old 40mm Summicron a lot until I drunkenly left it in a cab, attached to a lovely CL. Major reason I stopped drinking in fact. I also like Minolta's MD Rokkor 45mm f/2 and I pine for the Nikon 45/2.8 they put out a few years back. These days I'm finding that 45 is a little more vivid and muscular than 40. My final thoughts on this topic: people with the Lumix GF1 in particular as well as G1 and G2 users, worship the Lumix 20mm f/1.7 lens which really does appear to be outstanding; and of course on the m43 sensor that is essentially a 40mm lens. Of course when we're working in digital we do a lot of cropping so it's hard to say what a lens looks or feels like after we're done with it in p.s.