Bit late to contribute to this, but yes, both Mega X and Y are powerful. Whilst Y is better suited on Sun teams (quite clear due to the ability), X is an amazing setup sweeper, getting access to DD to boost attack and speed, whilst having a good movepool to back up its amazing attack stat. For attacking, go 252 Attack and 252 Speed Jolly, but it's better off suited to defensive: 144 HP / 252 Atk / 112 Spe.
As for movepool, attacking variants are generally Flare Blitz / Dragon Claw / DD / Earthquake
Defensive sets are generally Fire Punch / Dragon Claw / DD / Earthquake or Roost