For the most part, I have had a good experience with USPS other than a few negative situations.
Pros
The pay is excellent. Training is in-depth. Chance for advancement. Can hire yourself out to different post offices in need of temporary help. Access to medical, dental, and vision insurance. Usually hired in to work one day a week for a regular carrier and then work whenever they take a vacation or other day off. If using your own vehicle to service the mail route, you get EMA along with your regular pay.
Cons
Have to be available to work whenever called in (and could be the last minute). Very likely that you have to wait many years to move into a full-time position (if ever). High turn-over rate among subs. If hired to use your own vehicle - you have high wear and tear on your vehicle. You must keep your vehicle maintained and gassed up to service your route.