This card is really bad. If you drop it onto your bench at all during the game, then you just gave your opponent 2 free prizes. That's not to mention that Bouffalant can OHKO this card (and Bouffalant is played in both Keldeo/Blastoise and Landorus/Mewtwo/Tornadus).
People say that this card is good in Empoleon because it can counter Mewtwo. I disagree. I did really well with Empoleon earlier on in the format, and I found that Empoleon itself counters Mewtwo. Since it only needs one energy to attack, Mewtwo will hardly be doing any damage to you. If they attack you, you can simply just Max Potion the damage away and 2HKO them.
Hey, Mew-EX is a really cool card... to that 8-year-old scrub that goes to your league. I'd simply just recommend avoiding this card altogether unless you want to give your opponent free prizes. Overall, I'd give this card a 1.5/10.
the art is pretty cool tho