Avantages
A great way to meet people. Lots of downtime (if you're into that sort of thing). Good salary for Japan depending on what they subsidize for you (except consider the yen is really weak right now). I had great kids.
Inconvénients
Prepare to be the gaijin monkey for multiple schools and to have a genki expression while you're doing it. Also, make sure you fit into their stereotypes of your country and pretend to play dumb at any Japanese culture or language. Depending on the teacher's view of you, they can make your life a nightmare. You are a puppet to them in this job. It gets degrading and old fast. If you have something to do outside of work to make up for the work content than it's bearable, but if you're in the boonies where there's nothing to do and not a single other person in your age range prepare for doom. No advancement. Lackluster, minimal training. Not great for your resume unless you want to keep teaching.