Pros
I have liked working with Heart for the past 2 1/2 years. The school that I am currently teaching at has a great atmosphere, students, and faculty. I realize that every situation is unique, especially in Japan, but I would encourage current ALTs to keep a positive attitude about your surroundings and your employers. It makes a huge difference!
Cons
The pay is low. That's really it. I've heard some complaints from other current/former employees, some of which are valid. I encourage anyone thinking of coming to Japan to teach to thoroughly do some research online before deciding on such a venture. Some people, such as I, are enjoying it to the fullest. Others absolutely detest it. Find what is good for you (location, hours, etc), and decide from there.