I was a JET for a couple of years
Hi,
I was in Japan for a couple of years on the JET programme. Different people react differently to time in Japan. It also depends on where you get placed, who you're working with and whether you get along with the other JETs in your area. My experience was overwhelmingly positive - I met many people (Japanese and JETs), learnt a ton about Japan and the Japanese language. Plus I got to travel around SE Asia in the school holidays and still managed to save a bit of money from travelling further afield afterwards.
If you can handle change, are independent, are OK with being away from your friends and family, are willing to bite your lip (be diplomatic) about certain things and make a genuine effort to learn Japanese, then you're pretty well certain to have a good time.
A guy is doing a blog about his JET programme experience (he's actually in the same post I was a few years ago) - lots of bollocks, but some interesting bits too.
Check it out here:
http://dan.jaquette.net/blog/archives/2005_07.html
Cheers,
Jonathan