@yami4ct: Yeah, I know Mega Gengar has a crazy good ability, and I'm assuming the stat buffs are extreme.
It'll take time to see which megas shake out. Most provide interesting alternate options to standard sets. There's a big discussion on mega viability on Smogon right now. Some are sure bets to see play, like Gengar. Others are not as sure. Pretty much across the board stat buffs don't always outweigh the bigger stat buffs a hold item might provide.