I've played against
~Volbeatdown~, and he was using mega-Gallade. Showdown's save replay is bugged, so I recorded it. I'll upload it later. Basically, I let Crobat, my only Mega-gallade "check" die stupidly (he crit it, shrugs). It got to +2, and destroyed my team. It OHKd Standard Suicine after Rocks, Specially Defensive Forretress, and 192hp Zygarde...in other words, it's pretty strong at +2. The only reason I won was because Mienshao's knock off kills at like -3. (ps don't cry about standard crobat).
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 just finished a game against
dingbat, and I was testing out mega-bedrill. It didn't even make an appearance because his team was just full of threats (LO Starmie, CB(?)Champ, Mega-Absol. He got up SR, and with all the priority under his belt, there was no way beedril was gonna do anything.
So far, I've only had trouble with Mega-Gallade. I think Mega-Diance will probably be OU through usage, but I can definitely see Mega-beedril staying. It reminds of me a physical alakzam!
(i'll post links of replays when they are back up..and if not i'ma upload them anyways).