I just bought a CV 15mm for $380 CAD. Maybe I got ripped off, but I really like the focal length. It lives permenantly on my Zorki 6, now, since I don't have to worry about VF/RF accuracy on a Russian Camera anymore. Not that I don't like the Z6, and not that it isn't deadly accurate, but it would be a waste on the M2.
For street shooting, if the 50mm is the standard issue Colt 45, the 15mm is a sawed off 12-gauge. I loves me some wide-angle distortion, and composing right up close is a neat feeling.