Avantages
I've been working in Tokyo HQ, tech department. I do realize it's a big company and a lot depends on a team, department, etc. Your mileage may vary. For me, thought, it was 5 years of fun in most cases. I happen to join a very positive and dedicated bunch of people, a team working with mail system. I felt accepted, my skills has been put into a good use and developed as well along the way. My opinion mattered and I could have direct impact on the product we've developed. Obviously, not always you can start something from a scratch and not always you have super exciting features to develop. None the less there were a lot of challenging problems to solve - either new or existing ones. Company is big and so I've never been bored, there is always something interesting to do. It is a Japanese company, but very open for a foreigners. Almost everyone is speaking okay-ish English and if not you have someone who can help at the hand reach. Free food all day (if you come early enough for breakfast or stay long enough for dinner.) Combini shop on premises. Quarterly free team party. There are some annual events that you can join as a guest or host if you like to (tech conference, family day, group day + baseball match in Tokyo Dome, etc.) Plenty of activity clubs to join to. Gym on premisses. There is no dress code except of ID badge. Company closes offices for 3 days in summer and 3 days in winter to give people some rest time. No one ever asked me to work over hours, on the contrary, they are making sure you are not working too long.
Inconvénients
Salary could be better. There is a lot politics involved (again, big company) so you must be careful not to be tangled in it, unless this is what you like - I don't. There are combini shop and vending machines in the building (this is nice) but you must pay with company issued prepaid card only (this is annoying.) Sometimes you may feel a religious respect for the CEO - he's not a bad guy (if a bit arrogant and a bit boasting sometimes, generally okay-ish, though) but sometimes it feels like people are exaggerating a lot when they speak about him, about his ideas, etc. to the point that you may wonder haven't they lost a contact with reality. There is a cult of measuring everything in the company but it happens that wrong metrics are being measured for KPIs. Rigid Japanese laws means - no flex hours, no remote work (latter has been relaxed a bit though.) No sick days - you must use holidays or come to the office even when you're cold. Too many meetings. Some internal tools are terrible, old and no one wants to improve them. Benefits program is terrible. No more Japanese classes (they were there few years ago.)