Pros
If you're lucky, you'll have a decent office with management that won't hound you for every little detail of work. Once you're on the road, it's only you so you wont have anyone breathing down your back.
Cons
The position is physically demanding and many offices have management that will not follow the carrier contracts which then leads to union grievances. The pay takes almost double the amount of time max out. If you want to get into a position outside of delivery then it becomes about who you know, which translates into many internal lateral moves before applicants even get a chance to break free of being a carrier. Look up NALC pay chart for salary. Table 2 City Carrier and City Carrier Technician. The steps range from AA to P. Each letter is assigned a salary.