Pros
Good pay (~$16-18 an hour as an entry level temporary employee) If you make career it's a great deal. Good pay, federal benefits, 5+ weeks of vacation at highest level, you'll never get fired, etc. But it's hard to know when or if you'll get a career position somewhere, and in many places you could wait 10+ years. In other you could be promoted within a month. The modern Post Office is all about being in the right place at the right time.
Cons
Sticking it out as a temporary employee can be really rough, especially depending on where you're hired. Some places you will work 60 hours a week and never have days off. Some places will give you 20 hours and then you have to find other offices that need extra help if you want to work full time. Like most places, your bosses and coworkers will make or break the work environment. And in my opinion, the smaller the office the better.
