Funny but I am responding to your post from Osaka Japan. I have been with JAL for 7 yrs and really enjoy it so far. I am with HACS and they to have been very good with great benies. I think you are considering the ANA commuter contract or the JAL express B737 contract. Osaka is a big city with a lot of giajing living and working there. Lots to do and Starebucks are poppin up like pimples on a teenager. Depending on your contract you will probably do month on few weeks back home kind of thing.
You will get burned out on Japan after a while but is easily fixed with a break at home. The Japanese are very respectful and very kind to foriegners. Training will be very tough and very anal. You will be able to build your aircraft when you are finished. Almost all checklists are memorized with all items done from memory then read off the CK list. After training operations are very safe and very profesional. Most people have trouble passing a JCAB ( Japanese Civil Aviation Board) medical. I think that the standards are more stringent than NASA's. Many people pass the the Sim ck only to wash out of the PE portion.
In closing I think that you can expect a very positive experience in a very safe country. Pay and benefits are at or above industry norms. Like any company though it is up to you to make the best of it. I know some co workers have had a hard time in Japan but they are generally unrespectful of local customs and procedures. Give respect and you will get heeps back. Hope this helps and good luck.