Avantages
Working at public school can provide alot of down time which could be good or bad. In my case it allowed me to work on my own projects away from home. Will help with japan related problems if promoted to. In general teachers at the public schools are friendly and welcoming.
Inconvénients
Extremely low pay. Local School boards decide your ALTs pay, and dispatch companies like RCS take a percentage of this. This results in very low pay for the ALT, almost so low it makes it incredibly difficult to live in a city like Tokyo. Also sick days are not allowed, and if one is taken even providing you call RCS a day before, they will deduct that days pay, plus your monthly bonus. Rushed training didn't prepare me or other teachers for what teaching at a public school was really like. Also they don't cover transportation costs, which can really add up over a week.
Finaly if the school you are dispatched to is a low level school, be prepared to deal with some of the worst unmotivated yankee like students in Japan. Students sleeping in class, mouthing off to the teacher, and all around disrespect even during a fun and exciting lesson.