I really think a lot of it simply has to do with where you live and what the people who work in your area are like.
I've been an online retailer for almost 12 years and if there is one thing I've learned: people just have different experiences with various carriers based on their locations.
I rarely have a problem with USPS - I often get my Priority packages in 2 days and never more than 3.
I also like UPS a lot but I have customers who loathe them.
For me personally: I HATE Fedex Express - I have had them lie about attempted deliveries so they could meet their time guarantee and several times I have had them deliver to the wrong address (both wrong number and wrong street). They also pickup from Fedex Kinkos and their drop boxes whenever they damn well please no matter what the posted pickup time is.
Yet, I have customers who SWEAR by Fedex and pay more to use them.