I've never sent any post through Priority Mail. Generally, I've only seen pretty fast times when using standard mail.
Well, at least I won't feel a tinge of regret when I don't spring for faster postage. WSJ findings via Star Tribune
Having worked at the post office, I found that Priority Mail was treated exactly the same as other 1st class items.
It came in in the same carts, and was sorted at the same time. If it was slower at all, this is due to the fact that it is often oversized, and thus can't be left in the conventional mailbox but may need to be picked up.
Media mail/book mail and other cheap means of shipping items usually mean the delivery is send only via ground. 1st class/Priority and higher rates are often shipped by air, being loaded into airliners at the airport.
Bulk rate flyers and other junk mail is often just delivered on a pallet, and is tackled by the employees once the other stuff has been attended to.
While I didn't read the WSJ article, I'd be curious to know what they compared. If you're mailing something to a nearby destination, regular mail is usually quicker. But if you're mailing something from Boston to L.A., I'd almost guarantee that Priority Mail is much faster. This, by the way, is something the USPS has always admitted to. Also, the delivery guarantee is from post office to post office. So if you go to your local branch and mail a package to a friend in Oregon, they guarantee that it will arrive at his local branch in 2-3 days--they don't guarantee that it will actually arrive at the destination address in that time.
One thing I'd like to add is that this topic came up on the radio a few years ago. Several postal employees called in, and while they all agreed that Priority Mail was not always the fastest, they did state that it was generally much safer than regular mail. They stated that more attention is given it due to it being a premium service, and that carriers and other workers will usually take better care of Priority Mail packages than the regular mail. This prompted a quick call to the show by the local postmaster who claimed that all mail is treated well. Under pressure, though, he admitted that more care is given to the PM packages.