I often rave about it as the best way to come to Japan as an English teacher because it really is. I was on JET from 1992-95, then I was the JET Alumni President for my local Canadian chapter, and I was on the interview panel as well. I have seen a lot of people come on the JET program and it has been the kick-start to a lot of careers.
Of course there are those who say it was not a good fit for them, or that it was just something cool to do while they were gearing up for something different. And all of that is perfectly cool. JET is not meant to be a career, but it can be something that you can springboard off of.
I made a couple of videos on the JET program and I encourage hopeful newcomers to Japan to really explore this option carefully before just signing on with any ALT contract that gets thrown at you.