I'm well acquainted with these suites from many different performances on cello but never heard them played on viola before. Helen Callus' viola has a beautiful tone and resonance, and she is very well recorded. She plays expressively and movingly without taking undue liberties with the rhythm and tempo. William Primrose believed that violists should never play the sixth suite because of its high pitch. It was written for the obsolete five string cello, so cellists and violists today in the absence of an E string have to play far up on the A string, especially in the prelude, which is probably not the best range for the viola. Callus, however, transposes the sixth suite from D major to G major, so that what would have been played on the highest string of a five string instrument, the E string, can be played on the A string, the highest on modern instruments. This brings the highest notes into a more flattering range for the viola, and it also means that the resonance of open strings isn't lost in the transition to a four string instrument. Obviously the first choice in these pieces is on the cello but if you want an alternate version, I recommend this recording enthusiastically.