I was a JET for two years. It was a really good experience on the whole. Its really upto you how much time you spend with other JETs - you stay with them the whole time or have virtually nothing to do with them. The jobs vary a great deal from being a human tape recorder to having lots to do though a lot depends on how proactive you are in finding clubs and things to join in.
I recommend Shikoku especially Tokushima ken where I was (though if your a big Murakami fan then Takamatsu where lots of Kafka on the Beach is set is on Shikoku too).