Vantaggi
Secure job environment, with management that takes very good care of the employees. If you have problems (which, as a foreigner, you will eventually run into), the expectation is that the company will arrange things. This is incredibly valuable in the first few months getting of used to working in such a big company - and if you are new to Japan, there are many things you might need help with. Hitachi is involved in so many industries that there is a tremendous amount of opportunity within the company, and it is not uncommon to be moved about - both between locations and between roles. It is very interesting to gain exposure to many different aspects of the business.
Svantaggi
Although better than many companies in Japan, Hitachi is still stuck in the Japanese attitude that working longer hours means you get more done. But in some positions (such as in research), the work is never complete. The excessively long working hours and poor work/life balance that Japan is stereotyped for is true of Hitachi, despite efforts (eg "no overtime Wednesdays", which are ignored) to change it.