Most certainly late response, but I'll drop my 0.02$ anyway...
I did several half-formal family events with my M6 and a simple, cheap but yet reliable Sunpak 383 Super unit.
Once set properly in auto-aperture mode (or as sometimes it is called - auto-thyristor), it did beautifully, every bit as precise as I used to with a contemporary pro SLR systems featured by a sophisticated flash metering.
Just make sure you understand how to work with that combo.
Mind you, I only used that indoors, have yet to try that out for fine fill-in outdoors - this is probably the most tricky scenario for flash exposure, chances the most advanced SLR flash metering may have an edge here, but that have yet to be proven....