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> I use the SF-20 on my M7 and I find it a nice accessory. The size is good - small - and the flash provides great override features, and TTL as well as manual and Auto modes, just what I need for the rare flash shot. The overrides let me do fill-flash, which is mostly what I use it for. My only complaint is the lack of a high-speed synch (above 1/50 sec) but I am not willing to go to the MONSTER Metz to get that - I much prefer the small and unobtrusive SF-20.
I sometimes use the SF-20 on non-Leica cameras because the AUTO mode is very good. Also, the flash has nice batteries and they charge it up fast, even after months of disuse in my bag. It is a hard flash to beat for the M-shooter, unless you use a LOT of flash, but in my experience M-shooters use very little flash, so a very little flash works well ;-)