Pros
First: In this economy? The pay. New carriers start out at $15,30/hr and (even though your orientation leader may so you're not guaranteed 40 hrs/week) you will get a monstrous amount of overtime. Once you're past your first couple of months and you understand how to carry mail properly you will often work from 8a-6p nearly every day. Also with a few cities, like mine, you will work on Sundays for Amazon. This usually adds an additional 5 hours to the paycheck. Myself and other CCA's in the station work between 51-64 hours a week.
Secondly: You are your own boss for the most part. You will spend 1-2 hours a day in the office between receiving and casing your magazines and any left over letters that the machine didn't sort out. Once you've been in past the 90 day probationary period you are eligible to "hold down" an open route. If you are lucky enough to get a good long term hold (the regular is gone for injury or some other reason) you will learn how to case routes very quickly.
Third: Fitness. There's a lot of people who want to lose weight out there. I weighed 235 lbs when I first started working for the post office and now I weight 180. I lost 50 lbs in the first 3 months alone. It's all exercise though. You can diet if you want, but remember you'll need energy to walk those long routes.
Fourth: Coworkers. Yea, there are turds in every environment, but most of the career employees there are really pulling for you to succeed. Most carriers in my station are former military and a lot of them have been friends for decades. Being a CCA myself, I was worried about how well I'd fit in with some of the grizzled older carriers but they accepted me right away.
Cons
So where to begin. Well remember when I talked about working all that overtime in the Pros section? It's not optional. You will be expected to be at work every day of the week, including Sundays, unless you have a decent management staff. During the Christmas season I once worked for 53 days straight without an off day. We had new CCA's get hired and quit within weeks. Have a family? Tough luck. You will get to see them from 6:30pm till they go to sleep. Sundays you will likely get off work around 1-2pm.
Management is mostly compromised of people who are former carriers or clerks, which is nice because they promote from withing, but the devastating caveat to this is that most of them are uneducated persons. A fair amount of carriers start when they're in their late teens and early twenties and come from jobs that were minimum wage or did not require them to have any kind of leadership training. The managers don't care about the welfare of the employees mental status until it's too late, and most of them tend to act like they were never carriers at all by expecting completely ridiculous things from the CCA's and some career carriers. It's not unusual for a carrier to be given a 2 hr "assist" in addition to whatever their main route is. While most carriers can get this done without much issue, for a new carrier or even an experience carrier on a bad weather day, it can become very stressful mentally.
The threat of being fired is incredibly annoying as a CCA. If you call off sick, if you need to have a personal day, if you even need to pick your kids up from school because your wife got stuck late at the office, a manager will pull you aside and remind you of how expendable you are. The Paid Time Off (PTO) you accrue will come very quickly, and you'll soon realize you have 40 hours and would like a nice little vacation.. too bad you can't take it. As a CCA you're expected to work 360 days a year and then you get 5 days off as a reward and a massive paycheck AFTER your 5 days off. Now you can use that fat cash to...uhhh.. buy something I guess? Certainly would have been more useful if I got it before the 5 day period to use on my vacation.
While the career carriers are really great to deal with usually, the fellow CCA's can become very competitive. Often times if you're given an assist and it's better than another CCA's assist who has "seniority" over you they will complain to other carriers and management that they should have gotten the "good" assist. This is one of the fatal flaws that new people with struggle with. No matter how much faster you are, no matter how much more accurate you are, no matter what, everyone gets promoted by time with the post office. This leads to a lot of carriers just doing the bare minimum and putting the excess on other CCA's or carriers.
The final con (that I'll write about) is that the weather sucks. I know carriers who have been delivering mail for 20+ years and they still can't deal with the rain, the snow, or the heat. The heat is the biggest killer for carriers by far though. If you're in an area that suffers from hot, muggy summers, get ready to consume gallons of water every day, and sweat that out (often onto your customers mail). The worst is when it rains on a hot summer day and then evaporates right off your clothing. Makes you feel like a walking sauna.