I was offered a job instructing before starting my FI course.
Flight instruction seems to be great fun so far, and extremely rewarding. That said the wage is not enough to make it a long term prospect. I will have to use savings to subsidise my salary (working 60 hours a week, 6 days a week) and I know other FI's in a worse position at other schools. Unless this changes I can't see why anyone would want to become an instructor as a career choice. Yes flying is great fun, but you do need to live.
From other guys I've spoken to it seems two years is about all you can handle of basic PPL instruction full time.