I had the pleasure of knowing Joe Pass in the last days of his life, and he considered this to be his best work with Ella Fitzgerald. It is the only album they did together where he used an accoustic guitar, and the subtle shadings found here cannot be matched in their other work.
As for Ella's voice, it is the most controlled and inspired of their four studio albums. All of these albums were done with no rehearsal, giving both Ella and Joe a chance for true spontaneous improvisation. The song choices are excellent, and I especially recommend "Nature Boy" for the most unusual tune the two ever did together.