Ventajas
The JET Program has excellent benefits. You have what is considered a high salary, usually will get some form of housing assistance, have health insurance, your flight to and from Japan is paid (assuming you see a contract through) and sometimes many more benefits. It's also a fun experience to do with the ability to travel and live in a new country.
Desventajas
The staff in Japanese schools can and sometimes are passively hostile to the ALT. JET participants also have no true support system, this is a lie propagated by the Japanese government. If your school is mean to you then no one is able to help you. They get out of taking responsibility for a lot of the bad things that happen on JET by simply saying "Every Situation is Different". Doing JET also does not look good on your resume unless you are pursuing a career either in ESL work or desire to work for a Japanese company.