I haven't worked for the post office for years, but it wasn't too bad when I worked there. I started right out of high school and worked two years as a carrier. This was about 35 years ago.
I was a rural carrier on one route - mostly post office boxes with about 13 on the road boxes. I loved this route. It was perfect. Then I became a sub for a larger route. We are talking a 70 mile route, half the roads were dirt roads and half the dirt roads washed out during the winter. I would do this route on weekends and when ever the other carrier was off. She was off for about a month and so I was on it full time. I hated that route. I ended up quitting because it was just too stressful and tore up my car.
I went back and trained to become a clerk (working inside only) and that wasn't too bad. But I was also training to work on one of the sorting machines and got sick and could not pass the final testing.
My dad was a city carrier for many years and liked his job. My mom was a rural carrier and then became a clerk and eventually became a postmaster. She also enjoyed her job.
There are many different jobs in the post office, from rural carriers (use your own vehicle and get paid mileage), city carriers (use the post office vehicles) and clerks. There are also administrative positions too. If you are a carrier, you are checked every few years to see if your route has grown too big or has dropped in size. If it has grown, they will usually move part to another route. Clerks do a wide range of things, from sorting mail and packages, to working the front window (not all clerks do this), to handling forwards and vacation holds, to putting the mail up in the post office boxes at that office.
Being a carrier is not for everyone. Also a lot depends on the upper management for the area. Unfortunately there is a saying that to get someone out of an office, promote them out. So there can be some real jerks in upper management.