Unfortunately for us, USPS delivery around here is unreliable. Sometimes Priority Mail stuff shows up in the expected timeframe, but just as many times, and more so lately, I have had priority mail packages sit at "the next town over" for 2-3 business days after the tracking has shown it left the Raleigh sort facility. The local mail carrier lady told us the last time this happened that they are horrible over there (in Rocky Mount) - they get stuff from Raleigh and sit on it for days unsorted. And of course when the local mail carrier delivers it they are the one that has to deal with the customer. She said they have complained about it to their managers but nothing is being done about it.
We used to live in a different town closer to Raleigh (Clayton) and we were pretty happy with Priority Mail - heck, probably half or more of the parts etc. I bought to do a custom modded PC mod came Priority Mail and we never had a problem. Here? Well, if it's something I really have to have, I will look for somebody who sells it that offers UPS or Fedex service and buy it from them before I buy it from someone who only offers USPS service.
Another problem is the undercurrent of riff raff in this town. If you leave anything sitting in your mailbox very long, well, you may never see it. And this is supposedly a "nice" neighborhood. (We've learned over the last couple years that "nice" is a relative term, especially if you "ain't from round heer".).
None of the carriers are perfect but we've had a lot less trouble with UPS and Fedex.