I have found instructing to be a very valuable string to have on the bow. The course itself (depending on where/who you do it with) will force you to learn an awful lot about flying. I would go as far to say it is the second hardest flying course you can do behind a JAA Instrument Rating. Teaching itself can be very rewarding. Like anything, there are good days and bad, though generally it is good fun and helps pay the bills. The other thing to consider is that you are a lot more employable as an instructor/commercial pilot than just a CPL. If you can land a job within a decent outfit, it should open a few doors for you i.e. gaining commercial experience and type ratings depending on what type of outfit you go to. Looking back, I would still choose to become an instructor all over again. In terms of where to do the course - you need not look any further than Mike Green at Helicopter Services High Wycombe. The most experienced guy to do it with. Hope this helps.
FF01