Pros
Had a wonderful time teaching for most of my tenure on JET as an Assisant Language Teacher (ALT). Enthusiastic kids, great support from friendly staff, and I eventually gained the trust of the other English teaching staff to allow me to take complete responsibility for all of my 20 or so classes a week. Very rewarding Experience. A lot of down time. A plus if you're self driven, or looking to develop miscellaneous skills. My environment (rural Japan) allowed me plenty of opportunities to interact with the local community. I also had fantastic support from fellow ALTs living in the same area.
Kontras
Before my first Head of the English department left, I was reading and repeating passages from the textbook 30 classes a week, really boring work for a whole year. Most people on the JET program have mixed experiences with English staff.