When the M9 was announced a decade ago (gosh), it really had amazing image quality thanks to the CCD sensor coupled with those fantastic M lenses, which were arguably superior to EF lenses at that time (less distortion, better sharpness and contrast).
Now others, (especially the new mirrorless systems) have caught up with producing top-tier optics (with AF, but also with a bigger footprint), so it is more subjective. Leica IQ was, and still is excellent - but so is Canon, Nikon, etc. each with a slightly different look, but if that tells anything, I do very much like the look of certain Leica lenses adapted to the R.