I haven't used one although I'm sure that others have. Most people using older lenses enjoy them on the digital bodies for their 'faults' and character (that's why I use a 1960 Hektor 135/4.5 and a Summilux 35 v2, amongst other older lenses 🙂.
Regards using collapsible lenses, Leica warns NOT to collapse any but the APO 90mm f/4 Macro. I understand that one of the Voigtländer 50s is also safe, but I'm not sure which one.
The reason is that there's not as much room in the interior of the digital Ms as there is in the film Ms: the sensor stack is thicker and the shutter is closer to the lens mount by some distance. With collapsible lenses, it's best to put a semi-rigid sleeve around the collapsible section so you don't accidentally push the lens in and destroy the shutter and/or sensor.
G