Before I got my M6, I used Nikon F2, F70, F80, Olympus XA, OM1, OM2n, Minolta X700, XD7 and a Samsung P&S. Compared to all of them, the M6 costed a lot more and was a pain to use - harder to focus, compose, meter and shoot.
But after shooting at least 10 rolls thru each camera, the M6 gave me the best images - composition, exposure, resolution & character.
While slower, metering & focusing is always accurate on the M6.
But more importantly is the Leica glass, none of the other brands mentioned have the impact that it has on the quality of the images.
As I develop my own film, I noted that negatives exposed with Leica glass were of a different density from those taken with other cameras or other brand of lens (Cosina & FSU) mounted on the M6 - even those developed in the same tank.
Haven't tried Zeiss yet though.