My mum got me one last year for Christmas. Needless to say I haven't used it - she thought it was a great idea. I don't fancy increasing my wind-resistance / sail power that much - I'm commuting here, not trying to get blown across under a car.
Never heard of them but I'm new to all this. First thing that came to mind was a cape that Superman would wear, which would look mighty interesting riding along public roads. Can't see the benefits myself.
Back in the late 60s we used cycle capes and sou'westers for mountaineering because there wasn't anything else! Then someone called Peter Storm invented a garment called a cagoule, which made you sweat like a good 'un.