Pros
The benefits offered are really good. The Japanese healthcare system that employees are enrolled in is fantastic. Most contracts offer a good amount of vacation time. Regular work hours are 35 per week. The salary is the best for this type of work in Japan. Some JETs live in subsidised or free housing which helps with saving. The job itself can vary, but it usually is not exceedingly difficult.
Cons
There is not much standardization in the program. ALTs are generally expected not to make waves and are not given much support. There is no advancement and even after years of working in the role the ALT is still seen as the lowest in the hierarchy. Despite the contract stipulations many ALTs are forced to work a lot overtime without compensation. Despite the program being advertized as a cultural exchange program many schools do not really care about this and do not know what to do with the ALT. Schools can use ALTs any way they like which can mean anything from having the ALT solo teach 6 classes a day with no planning period to the ALT sitting at a desk all day. Many ALTs wind up desk warming and wasting time instead of being included in something productive. The workplace system is extremely hierarchical and since the ALT is the lowest it is generally looked down upon if the ALT takes initiative during desk warming time. Many JTEs do not want to work with ALTs or are envious of the ALT contract. Your experience depends almost entirely on your JTEs and how they view the ALT role.