Hmm, I've thought about a cape in the past (I have a J&G vented jacket that I love) but I hadn't thought about the aerodynamic downsides. If I'm riding in the rain, it's almost always in fairly high winds, usually a headwind, and quite often gusty crosswinds that are already sometimes difficult to maintain a straight line in. I think maybe I'll stick with the rain jacket. Mostly when it rains around here it's warm enough that I don't really care if I get wet anyway.

Also, even with fenders and mudflaps, my legs and bike get really filthy from riding on the gravel roads, so I don't think a rain cape would keep those dry and clean, since that is coming from underneath anyway.