Can't speak about the classic, but the Leica flash on my M6 TTL works pretty good. Better when placed on a Nikon SC-17 cord so the flash is off the camera.
With a 1/50 flash sync, it's not great for fill flash on bright days.
I don't use the Leica flash much with my M6 TTL. Prefer using an ultra fast lens instead with shallow depth-of-field in dark situations.