Probably the World's best viewfinder for manual focussing (well, beaten by the Leicaflex SL).
Incredibly well-dampened mirror & shutter (despite not having a mirror pre-release, it still has less vibrations that a Nikon F or F2 with a raised mirror). Rivals my M6 for slow hand-held shots.
Very tough & reliable.
Film advance advances only if film itself is advancing (and counts down when rewinding, making leaving the leader out very easy - mid roll changes are very quick).
The lenses are pretty good... ;-)
Intrmediate shutter speeds work.
Shutter lag very short for a SLR
Shutter speed dial very smooth and nice to adjust.
I used to love my Nikon F & F2, but they can't compare with this beast.
Second favorite; Leica M6, for the usual reasons.
Third favorite; Nikon F & Rolleiflex SL3003 tied for very different reasons.