I have had ongoing battles with the USPS in Puerto Rico (which, for those who don't know, is part of the U.S. and uses the USPS) for the last few months. The so-called "2-3 Priority" inevitably takes 4-5 days. This wouldn't even bother me so much except that on the tracking information, the "scheduled date" (which is NOT guaranteed) is always the 3rd day after a package was sent. USPS will then change the scheduled date on the tracking information to make it appear that they are delivering things in a timely manner. To make matters even more irritating, First Class to Puerto Rico takes exactly the same amount of time as Priority--an expensive lesson I've learned in an effort to get my packages a little faster. I recently wrote to USPS and told them that even offering 2-3 Day Priority to Puerto Rico is ethically suspect if they don't even make any attempt to get Priority packages to people faster. (Okay, I'm done venting. . . I needed that!)
Needless to say, I'm no longer upgrading to Priority Shipping.
Added later: It occurred to me that I didn't say anything about the fact that I know Priority is a bit faster--not necessarily a lot faster, though--at least in the contiguous 48. I suspect Hawaii, Alaska, and the other U.S. protectorates also have problems.