Here are my two cents about Diancie, first the abilities.
Clear Body: Stats cannot be lowered by your opponent's moves. Useful in case you don't like getting splattered by an Unlucky crunch defense drop or something like that.
Magic Bounce (Mega Diancie): Status moves that target the 'mon with Magic Bounce are reflected. Magic Bounce is a brilliant ability, and on Diancie it's no exception. Not having to worry about Burns, paralysis or sleep is great, nor entry hazards or leech seed.
Then stats...
HP- Base 50: Not good. Base 50 is Equal to Deoxys, meaning it won't be walling anything for ages.
Atk- Base 100: Pretty good. Base 100 isn't stellar by any means but it's still pretty good. Mega- Base 160: Good lord. You don't wanna mess with this thing. While Diamond Storm and Stone Edge are the only two moves Mega Diancie gets that utilize this amazing attack stat, they're still powerful moves.
Def- Base 150: Diancie's low base HP is forgiven. This means that Diancie, while not being able to wall, can solidly take a few hits before going down. Mega- Base 110: While losing a stat as a mega can be Catastrophic to its usability (LOOKING AT YOU, GARCHOMP), the defense drop doesn't actually affect Diancie all that much, considering it's still above base 100, meaning it can still take hits rather well.
SpA- Base 100: Base 100 SpA is basically like Diance's Attack, except it gets much more usage from it with Psychic, Psyshock, Dazzling Gleam, Moonblast and Earth Power. Mega- Base 160: ...And of course, that applies to Diancie's Mega too. Base 160 SpA is stellar, and Diancie has just a good enough movepool to make it work.
SpD- Base 150: See Def. Mega- Base 110: See Def.
Spe- Base 50: In its regular form, it's rare to see a Diancie go first, but then again, it is basically Carbink with better offenses, so that's forgiven. Mega- Base 110: But Mega Diancie's base speed is nothing to laugh at. Considering its stats leaning towards offense, having speed that rivals Latias and Latios is brilliant. just brilliant. It means it outspeeds Garchomp! That's an excellent speed tier to be in.
Now then, onto my set...
Diancie is a Girl's Best Friend
Diancie@Diancite
Nature:Rash, chosen because physical steel-type moves are more common than special ones.
EVs: 252 SpA, 128 Atk, 128 HP, or something similar.
-Diamond Storm
-Moonblast
-Earth Power
-Rock Polish/Calm Mind/Substitute/Protect/Psychic
Offensive Mega Diancie set, preferring Special attacks. Diamond Storm is the obligatory STAB and a pretty good physical move, plus one of the only reasons Diancie actually has to run mixed. It also helps that it has a chance to boost Diance's already great defenses. Moonblast is a Special STAB, and the SpA Drop can help you in the off-chance that you get it. Earth Power is necessary coverage on Diancie as it allows it to demolish most Steel-Types that would otherwise crush it otherwise. The Fourth Slot can be a manner of different moves. If you want to sort Diancie's not-so-good speed (Though i don't see why you'd want to do that on Mega Diancie), Rock Polish is the way to go. If you want to make it hit harder and take hits better, Calm mind is a viable choice. Substitute can only really be used for taking damage. Protect is obvious. Psychic is if you can't decide what you want to run and decide you want that little bit extra neutral coverage.
Counters and checks:
To counter Diancie, just throw a relatively good Steel-type move its way and it falls. It's basically Carbink with better offenses, so most counters to Carbink should work, unless Diancie has coverage moves, like Earth Power. Garchomp threatens to wreck Diancie with its deadly earthquakes, but has to worry about the Fairy STABs, and can't even consider coming in on Mega Diancie, as Mega Diancie *WILL* outspeed and OHKO. Excadrill is another good choice, but, like most Steels not named Skarmory or Ferrothorn, has to worry about Earth Power. Speaking of Ferrothorn, Ferro wrecks Mega Diancie with Gyro Ball, due to Mega Diancie's much higher speed.