Vantaggi
I have worked at Minerva for almost 4 years now. I have also heard the seemingly endless roll of horror stories, from managerial incompetence to pay disputes, about other teaching companies in Japan; Minerva embodies none of them. Admittedly, I am rather independent and self-directed, so I haven't needed much support, training or otherwise, from the staff. Nevertheless, when I have needed things, the staff has been astonishingly prompt and cordial.
Svantaggi
The training has been unstructured on the occasions that we've done it. I have seen posts elsewhere from ALTs belly-aching about this at other companies, so presumably this is important to someone. However, if you need someone to train you to be an ALT, perhaps you should pursue other career opportunities (Tip: Don't aim high.).