I recently picked up a Nissin 622 ii flash, which has the dual benefits of TTL and wireless (and both!) with my Canon 7D, while also having a PC sync and full manual mode. I hooked it up to my Rolleiflex with a flash handle and soft box, and my better half said "what is that? Those just don't look like they belong together!". However, the negs tell a different story, they look great! I might consider trying the flash with my M6, but it will be a bit top heavy, and the Leica still has half a roll of Tri-x at ISO 800 with the C-Sonnar on it, hence no real need for it's own light source.
Cheers and Happy Holidays everyone,
Rob