Pros
My schools, the community, and the lifestyle provided me with the 3 best years of my life to date. I loved my kids and co-teachers, had a great ALT community where I lived, and really enjoyed the small town lifestyle while I was there. It was a truly transformative experience that I would recommend as long as you know what you're getting into and have a positive attitude.
Cons
JET itself basically pawns you off to your city or prefecture and goes hands-off from there. I had an amazing experience mostly due to my own hard work and the wonderful community I was surrounded by. While I know this is true of every job to some degree and certainly heard more success than horror stories, I know a lot of people who had crazy circumstances and were given little to no support by the program itself. It is far more likely than not that you will have a good experience, but too much variance for me to give a solid 5 stars.