my own experience is that when you first instruct its great, being paid to fly for a living, it feels like the best job in the world.
but somewhere around 1000 hours instruction it does start to do your head in and can feel mundane with the endless cycles of flights last just about 1 hour, and you start to resent working so hard for such unpredictable but never high returns
you give so much of yourself and sometimes get so little back, as a full time job i dont think its good and agree with whirlybird that as an add on to something else, whether that be charter or some other flying job or even something non-aviation its fine.
CF