Pros
The pay, the benefits. If you like walking, seeing new communities and neighborhoods( even in cities you've lived in for years and thought you knew well) being a letter carrier is great. Since you also work alone, you don't have to talk to a lot of people and can be alone with your thoughts. As an ex-college student whose primary area of interest was English, it's amazing seeing all the newspapers, books, and periodicals that are out there. Occasionally, I'll see a magazine for some small or esoteric subject and I'll think, "my god, they have a magazine for that, too?" The job can also be entertaining and informative. Sometimes I'll see an article posted on a magazine cover and then when I get home I'll look it up online (I think people should receive magazines and newspapers they've subscribed to fresh, new, and unread, so, no, I don't read patron's magazines; if I can't find the article online, I'll either buy the magazine myself or it just goes unread). Finally, what other job can you look at covers of soft porn (Frederick's Of Hollywood, Victoria's Secret, Venus Swimwear, and the Playboy catalog) all day and not get fired. Oh, and one last thing, the National Association of Letter Carriers (NALC) is an excellent union. If you become a letter carrier, join it.
Cons
Hostility between labor and management is epidemic at the USPS. This will probably be more or less true no matter which post office in the US you work at. Management manages almost solely by numbers(mail count, time the carrier leaves the office, etc) and not through level, well thought out exchanges of information with the workers on the floor. Based on the mail count (which doesn't cover packages or full coverage mailings- mailings which go to every address on the route), they have a computer program which tells them which times you should leave and be back. Carriers can spend a great deal of time arguing with management why the information, or more often, the expectations from it, are wrong. And when you don't perform according to managements' wishes or the computer's data, it's your fault, not managements' or the computer's. There are other things, too, but I don't have time to go into them more. To me (and to a lot of letter carriers) this is the big one.