Instructing is a great way to build time and experience. As with most things in life it depends on why you are doing it and what are your long term goals. If you want to go Commercial and enter an ailrine I think its a great way to build up time, but try and limit the single engine time, multi engine is what you really want. If you want to do it for fun then yeah go for it, see it as a hobby where you get paid. The income is absolutely disgusting. I'm a qualified flight instructor, Frozen ATPL, MCC and I'm instructing to build time and experience with a view to entering an airline. I instructed full time during the summer and am now going part-time over the winter months so that I can go and earn a decent living by getting a full time job. Over the summer of 2006 I managed to get nearly 300 hours which takes my total time to 500. Not great in the big scheme of things but its better than just another 250 hour Frozen ATPL with MCC. Pay wise I'm on £12.00, bus drivers earn more than this, so I wouldn't consider being an instructor job to live off. The guys who do this full time at my school either have very deep pockets (and I mean DEEP) or a partner or parents to support them. Overall I enjoy instructing and its a great way to meet people(network), get some hours and keep your hand in on the flying side of things. I'd be happy to answer any questions you may have.
Smoothk