I've had no problems in the UK with post, and serious problems in the US. In one place I lived, my weekly delivery of a magazine I subscribed to regulary went missing, so much so that I had to have delivery officially tracked at every stage of it's journey (!).
Worst of all was that I used to mail my rent check. I didn't use special delivery and simply mailed regular mode as one does with any bill payment, expecting it to get there and it usually does. Except that one day I got a routine "three day eviction" notice in the mail, from my landlord/rental management company. When I checked up on what the hell was going on, they said they had not received that month's rent check! It never did show up, was never cashed, nothing (I put stop payment on it anyway, at a $20 bank fee...)
I was also always shocked at how expensive it is to send a package from the US to the Uk, regular speed/pricing, taking a week or two, not overnight or anything special, compared to how cheap it is to mail the same weight of package to the US from the UK. Fifteen bucks for a something light while my UK friend mailed something much heftier for a couple of quid, so I noticed on the franking mark. Many a time.
I'm much happier with Royal Mail service and pricing than I ever was with the USPS, which in my opinion stands for Uncaring, Slapdash, Pricey, and Surly.