Mirror Room // Fractured Realm
Perhaps to the point where someone felt it'd be a good investment to pay over $100,000 for one, although it seems doubtful this particular Mox Jet will ever be removed from its protective case
Combined with Manifold Mouse, Heartfire Hero, and other cards, Monstrous Rage became the single best combat trick in the game, if not ever, and understandably so
Back in 2000, her popularity was so immense that when the card was first introduced to the TCG, it was done through a national tournament
That could change as time goes on, but it looks like that will probably have more to do with the overall Pokemon TCG market than with the rarity of Mew ex itself
Good thing, then that Urabrask was well up for betraying his brethren