Sunpak 383 Super is quite non-expensive yet robust and reliable, works great on my M6 (non-TTL). Provides very precise exposures in its Auto-aperture mode (Thyristor), boost bounce and swivel head.
I did several events with one on M6 indoors - yielded excellent results, probably on par with expensive Canon/Nikon flash metering technologies.
Having said that, what I would expect the edge to a contemporary flash metering technology over M6/thyristor combo - outdoor situations where a gentle fill-in would be necessary to open up shadows. Haven't yet tried my M6/Sunpak combo in such situations, so no personal experience to share with.