I love my old Martin 0021. Mary (my wife) has one too and we occasionally play them together.I first played a 0021 in the early fifties. Bess Lomax Hawes had one that was either given or purchased from Josh White. I fell it love with it immediately.
The 0021 has pretty much been my first instrument. It has a good bite for accoustic rhythm jazz guitar (ala Hot Club) as well as a nice finger picking balance. I like it because it's not too bass heavy but you can lay into it under other instruments such as loud banjos and it will have presence.
It's a great accompaniment instrument for singing also. I use a Sunrise pickup in mine when we do small venues requiring some amplification. Sunrise is one of the best but I understand it sounds even better with an expensive pre-amp.
The 0021 was one of Martin's first made instruments. They seemed to always have it on hand when there was a request for it although you don't see many today.
Frank Hamilton