I dont have great diancie centric replay, but I will post something later on tonight when I get home. I think at least one of my replays shows diancie's potential.
Edit: Here's a decent one of my recent endeavors.
http://replay.pokemonshowdown.com/ou-142407729
Don't mind the poor play on both sides and the pretty shitty teams. Diancie has an untimely death to tangrowth, but I just kind of assumed it was ready made for trick room. While not overly powerful, it takes hits from the porygon 2 pretty nicely and does decent damage considering Porygon's massive bulk.
http://replay.pokemonshowdown.com/ou-142142124 This one is a bit better. My opponent got lucky with diamond storm missing, but without any physical defense evs, I comfortably took an earthquake from pinsir although it seemed like I wouldn't have taken a swords dance earthquake. Heracross easily cleans up late game.
So, not amazing, but it works fairly well on bulky offense as a stealth rock setting mixed attacker. I guess I hope it goes to UU though to be honest. Outside of baffling conkeldurr, flying spam, and charizard X, it really isn't that great, but not as terrible as many would have you believe though.