If I launch 10 attacks in a game, 2 might be with Mewtwo. I don't heavily rely on it, mostly due to the fact that it takes a bit of oomph to get the thing organized.
If they start tanking Keldeo, you need three energy on a Mewtwo to KO (if they have six, which they usually do) which typically entails dropping Ho-Oh. There goes the game.
I own a MiniRay and like the idea, but it just made the energy line too streamlined, making Ho-Oh even more mediocre as an attacker.