The priority tag certainly had no effect on my bag getting on our ORD-IAD flight. 1P, flying in F, checked in 1.5 hours early, and low-and-behold: no bag on the carousel!

I have plenty of nice things to say about United, but I was rather upset that afternoon (more embarrassed than anything because the in-laws picked us up and I'm standing there like a jackass for 45 minutes waiting for a bag to never arrive). It finally turned up on another flight, but late enough that they didn't deliver it until the next morning. The courier also threw the bag out of the back of his truck before bringing it to the door (this was over Christmas and there were presents in it). The perfect cherry on top to what was supposed to be a simple trip.