Hello, a friend of mine just completed a post-doc in Japan. It really depends! firstly, on how flexible and adaptable you are and whether you can fit into the Japanese system. I don't know where you're from but the Japanese society is very focused on etiquette and social hierarchy - and if you are planning to stay there for at least 2 years (typical duration of a science post-doc) you need some Japanese under you belt. (and there's adapting to the food as well)
If you're cool with all of that, Japan is a brilliant place for a post-doc. I would say it's 3rd leading in the scientific field after US and UK and an enriching place to be.