Pros
The US Postal Service has good benefits and good pay for the type of work required. It's a government job so you can get all federal holidays off plus a pretty generous vacation plan, especially for former military or previous federal service. I would rate the maintenance side of the Postal Service much higher than the mail processing side, it's much more laid back and a lot less stressful than having a manager looking over your shoulder making sure you are going to make you production goals.
Cons
Management is not the best and most managers are not really concerned with the employees needs and are just concerned with meeting their production goals. Communication between upper management and middle management is very poor and that makes communication even worse from middle management on down. There is also little room for advancement as most jobs are a specific level and only advance in pay as your time in service increases. You can advance but it is difficult and very competitive.
