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Megas have an interesting concept and seem to have good intent for implementation. However, execution was a bit poor.
I liked that it took some Pokémon that fell victim to power creep and made them more viable. Same for seemingly perennial shitmon. Giving megas to Pokémon that were already cemented in "the meta" seems like an odd choice. I get that popularity played a role in their decision to give some Pokémon the ability but some are unnecessary and pushed capable Pokémon into the broken territory.
Others, like pic related were done well. Still, I feel like Mega Mawile, Mega Houndoom, and a few others would have been better served as actual evolutions. Mawile, for example, was done really well. It was given an ability, star spread, (retconned to fairy), and logical design. If more cross gen evos were done like that, the eviolite meme wouldn't be a thing.
Regardless, it's an interesting feature. I prefer the regional forms gen 7 gave us to a temporary evolution form but whatever. If we see more in the future I wouldn't mind.