Vantaggi
They are a placement agency to find you a teaching job in Japan. They try their best to get you prepared with many orientations before you leave for Japan and immediately after arrival. Everyone employed through this company gets the same wage, regardless of experience, which increases the longer you stay. So this is a perfect job for first time career seekers. You can request a placement and many people get one of their top three choices. The job benefits are amazing and the experience is life changing.
Svantaggi
Because you are not employed by the JET program but rather a local government office in Japan, everyone's living benefits are different. One person may pay $800 in rent while another employee pays zero. Stipends might be different amounts. One employee may get extra vacation time. It all depends where you're actually placed. If you have experience the mandatory orientations can be painfully simplified and redundant. There is no opportunity for advancement, and you will probably need to fight to prove your worth at first. The English education system is a bit screwed up, but they are working on fixing it.