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The basic gameplay is top notch, some of the best in the whole franchise.
I also think it sells the themes of adventure and living alongside Pokemon and nature better than any of the other Pokemon games.
Just borrowing the Pokemon you need in the moment is cool and feels kinda MacGyver-ish but it was a little disappointing that you can't build long term relationships with them. The second game kinda fixed that by having the sidequests to get new partner Pokemon, which is one of the reasons I think it's better than the first in general.
My main issue with the games is just how story focused they are. The problem isn't that the stories are bad; I really like the way they unfold from helping old ladies across the street to catching Legendaries to save the world.
The issue is that they're REALLY story focused. You can very easily go a half hour or even a full hour without doing much gameplay at all, just walking around and talking to people. There's nothing exactly wrong with that but the game's main strength is the gameplay- if you've only got an hour to play and you're looking forward to doing some really cool captures and leveling up you can easily come away disappointed.
It also has much less replay value than mainline because you can't play again with a different team.
They're good games and definitely better than mainline in many aspects but they aren't exactly playing to the strengths of the series.
As long as you go in expecting nothing but a fun game with Pokemon in it I don't think you'll be disappointed.