Try using Weakness Pass Celebi + Diancie on late game, let us just say it becomes very fun. That said without boosting it isn't exactly capable of plowing through bulkier teams but it does have enough fire power to 2HKO most of them with the appropriate move which is where its strength shines IMO as I find it to be just plain disruptive as teams scramble to pick the proper switch. It is definitely more prediction based against defensive teams. Though what I do like is that she remains useful even against such teams because of Magic Bounce, and having mixed offenses to keep walls on their toes. On offensive teams, aside from obvious scarvers and surprise steel coverage, she has no issues plowing through them.After playing him, I knew that I could play with the new Megas. Diance is alright. I use it with sand (hippo) and use a -special defense nature since the sandstorm boost makes up for that...I think people forget that sometimes! It can set up on a lot of offensive teams, but it really does lack the power (+spa nature) to plow through fat teams. Though the use of diamond storm mitigates it and makes it an excellent mixed sweeper + defensive booster.
I am a bit hesitant with MDiancie on a sand team since she doesn't add any real defensive synergy to the team, and can actually compound it. A team with a good FWG core is usually enough. In UU I'm finding Rotom-H and Coballion to be great partners. Aside that she works well with Hydregion+Jirachi, filling up the Steel/Dragon/Fairy core.
We can hope OU doesn't take her but does seem like OU will be gobbling up a bunch of megas come first cycle, if the unofficial ORAS OU stats are anything to go by.