The situation you're thrown into can vary a lot. Whether you're in a high school, junior high, or elementary school, and whether you're in a city, suburb, or inaka/village area. And you can get placed almost anywhere, from Hokkaido, to a tiny island, to Osaka/Kobe/Kyoto.
There are some areas you absolutely cannot be placed, like most places in the Greater Tokyo Area. So no Tokyo or Yokohama. These places are filled with enough gaijin and private companies to fulfill their needs for English speakers. Also, no JETs in Nagoya, since another company- Interac- services most of Nagoya.
I've read and heard a lot of nightmare stories. Bad kids or bad faculty. Some experiences are inevitable, because you're a foreigner. But, especially from my ALT friends, I think it comes down to your attitude. You have to find a way to make it work. Some situation just blow, but you gotta roll with that. I mean, you're getting paid a decent amount of money (plus insurance, and often with subsidized housing) to do some very interesting work that- all things considered- isn't very difficult.
If I had language skills I would totally be down for an exchange.
But I don't, and I'm not a qualified teacher of anything, so SOL there.
Nah man, no teaching experience required. Also, no language ability required either (though it does help you). There are plenty of stories of JETs with zero relevant experience getting accepted.
Oddly enough, they don't want certified teachers, since they'd probably feel marginalized in the setting.
Lulz, when I saw the thread title I thought you meant JET. Now THERE is something I could go for. 
Hmmmmmmmmmmm, yes please?
