I have no reason to doubt that Martin is a very good teacher, and I certainly respect his confidence in his method. But I would like to respectfully disagree with the assertion that wether a teacher is a fulltime teacher or not is the most important consideration. The fact that a musician is an active player as well as a teacher is most certainly not a handicap as far as his/her teaching ability is concerned; it can actually be quite an advantage, for many obvious reasons. Fine players with "a few students on the side" can be, and certainly in the case of the player/teachers that I mentioned, dedicated teachers who teach because they love to teach, as well as play; and implement very thoughtfull and creative teaching techniques which are enriched by their extensive professional playing experience.
Marakanetz, I don't know wether you are a beginner or not, and what your long term goals for your guitar playing are; but I would certainly take very seriously Martin's advise that you look for a teacher with a solid and well thought out method like he himself has. But based on my experience and the experience of most musicians that I know, you should not rule out a potential teacher because he happens to be an active player. After many years of study with a variety of teachers I cannot think of a single teacher that I have had who wasn't also a working instrumentalist.
Best of luck.