An SF 40 I purchased used is unreliable. It will no longer flash after a few days of light use - died during an afternoon Thanksgiving celebration. Fortunately, I had a Canon EOS 5D mk IV and 430EX III-RT flash to use for flash pictures. But it was very frustrating that the M10 and its TTL flash didn't keep working.
Everything lights up and the head zooms manually, but no flash, either from an M10's trigger or the open flash button (the LED ready light). Changed batteries to a freshly-charged set when I got home - still doesn't flash, even though the LEDs light up
So bottom line - SF 40 may NOT be up to professional standards of durability, and you may want to avoid it for TTL flash with Leica. Admittedly this flash was (lightly) used, but it's still disappointing. Seems like the company with the best optics in the world has only unreliable TTL flash.
I used to use a Metz 32MZ-3 / SCA 3501 for TTL flash with an M6 TTL and M7. It worked great on those cameras, but doesn't work with the M10 (or my old M8). And used Canon 430EX version 1 speedlites have been rock-solid on many commercial shoots. Just seems like Leica can't get its flash act together. *** Does anyone have any suggestions for a reliable TTL flash for the M10? ***