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I think if you are a kid they can be fun. At face value they are can hold your attention. The fun comes from making yourself a niche within the game to amuse yourself. You can catch them all, breed perfect pokemon, create a perfect competitive team, or just make a team of your bros to rampage through the game. The game is great for allowing people to make their own game within a game.
The game is great at rewarding you at the optimal time to make all your grinding, repetition, and teadious efforts worthwhile. All the delayed gratification makes the payoff worth it and fun.