Frankly, I still don't buy into the notion that Greninja hurts Mewtwo's chances... at all. The notion that 6 pokemon slots is too much? Please, we had 6 pokemon characters last time. The fact that they take up 6 individual slots should NOT be a big deal.
Now onto the actual reactions... Before greninja actually had the "Greninja Makes a Splash" I didn't really think Mewtwo at all. It's hard to explain my thought process because I have no god damn idea but I had no trouble figuring out it was supposed to be greninja. I just didn't really feel the WEIGHT of the fact that it was greninja at all until he was actually revealed into the light. After that... I was just like, "Looks cool." For some reason greninja just didn't catch me off guard at all. When Sakurai mentioned cutting the transformations I had the thought in the back of my head saying: What about Pokemon Trainer? And this answer only seemed natural... but I sort of wish we could have grass starter representation as well... but whatever.
So yeah, basically all I had to say about it on the first viewing was, "... cool."
Still, I definitely feel like Jigglypuff+Mewtwo both making it in is VERY possible at this point so whatever. As a pokémon fan, greninja is EASILY my favorite of the 4 currently revealed (Actually, I don't really like Pikachu, Lucario, or Charizard at all really.) And as a playable character, he's also the poke vet I'm most likely to consistently play as. So far, all the newcomers seem unique and bring innovative ideas for a moveset but greninja seems comfortable bringing his own unique spin on pre-existing ideas (I kind of hope substitute is more than just a standard counter, probably will be though.) But I can't really say I dislike him for it, he makes himself stand out a lot in spite of that. Still, the puff and mewtwo have a good chance of eclipsing greninja for me in the long run...