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I like them overall and I'm quite interested to see what they'll do with them in the future, but it should have primarily been a way to buff up weaker Pokemon, like Ampharos or Mawile, instead of making already strong Pokemon even stronger, like Blaziken or Scizor. Later generations need to balance it out more.
Also, it was really weird how Game Freak promoted the hell out of them, and then made a lot of them post-game exclusive, only visible for an hour each day, and hidden in really obscure locations. Making a certain Pokemon exclusive to one version and the Mega Stone exclusive to the other was some bullshit as well.