Can't speak about the classic, but the Leica flash on my M6 TTL works pretty good. Better when placed on a Nikon SC-17 cord so the flash is off the camera.
With a 1/50 flash sync, it's not great for fill flash on bright days.
I don't use the Leica flash much with my M6 TTL. Prefer using an ultra fast lens instead with shallow depth-of-field in dark situations.
Good luck.
__________________
When I grow up, I want to be a photographer.
I know I shouldn't admit it in public, but I have been known to use a flash on my M6, although I do prefer not too if at all possible. I use a venerable Vivitar 283 on a bracket and usually bounce it off the ceiling with a fill card on the flash. Works quite well.
Richard Wasserman
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I use both my Metz flashes, the 44 MZ-2 and 54 MZ-3, on my M6 TTL and my Bessa R3A with both flashes set at either automatic or TTL and get wonderful images! Below is an example obtained with my 54 MZ-3 and my 50mm Summicron lens of our new cat.
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....
back in the good old days -shooting fashion- i used to use a wee tiny peanut of a flash to trip the slave unit on my main flash & umbrella rig. at the distances i was working it added negligible amount of additional fill and freed me from being tied to a **!%***@+!!!! sync cord. on my budget it worked great.
i did have to overcome something of a "flinch reaction" to seeing the flash in the viewfinder. with an slr - except for a either my canon pellix or the (then) newer EOS RT - i never saw the flash as a result of the mirror swing during exposure. funny i never noticed that effect with my oly xa...hmmm.
enjoy the flash and experiment...using mainly cameras with leaf shutters when i do use flash now i miss the effects of leaving the shutter open (lower shutter speeds) and all the neat ambient light you can pick up with a focal plane shutter.
In my camera bag there is always a Metz flash. I don't use it often, but my M6 without TTL is very happy with it. Indirect flash (ceiling) also works fine, I never missed the costly TTL version.
If I were a busy 2008 press photographer trying to survive competition I would own a digital EOS with a dedicated Speedlight flash, that's a "fire and forget" combo that always creates good results, no matter how and where you use the flashlight, in darkness, as fill flash... good stuff, but that's for WIMPS.
Have you used a Leica M6 with Flash Unit 
Linear Mode
