I've had good experiences, mostly, but also bad, with USPS.
The experience that annoyed me most was a few months ago when a buyer sent via certified mail, signature required, a check for several thousand bucks for a Korth revolver I'd sold him. I was tracking its progress online, and was sure to stay home on the day it was expected.
Late in the afternoon I received an email notice from USPS stating that delivery had been attempted but that no one was home. Since I had been there all day waiting, this seemed improbable.
When I checked my front door, a USPS attempted delivery/nobody home sticker was on my door. It stated if I wanted my letter I would need to go to a particular post office and sign for it.
I checked my security system doorcam to see what happened. I saw the postman had approached my door, lightly knocked on the door frame — not the door, the frame, so that no one could hear — ignoring the doorbell, and then immediately, without waiting, stuck the prewritten "nobody home" attempted delivery notice on my door and beat feet.
I went to the post office the next day to get the letter. Took me about an hour out of my day.
Then there was the time it took me about ten months to get back the insurance money for a holster that USPS lost... Turns out USPS subcontracts out their insurance coverage for Priority Mail, and the subcontractor was hard to reach.
But, hey! As I say, most of my experiences with USPS have been good...