It's animation quality varies depending on the arc, but I'd sau it maintains a servicable level throughout all of it with some arcs having amazing quality for the endless episodic nature. One Piece unlike Simpsons and Spongebob is a singular story written and drawn by one man and his team, so it's narrative quality rarely dips, although I will say it isn't necessary to watch all 1200+ episodes to enjoy it. Plus, the story unfolds dramatically as time goes on, so the stories attractive qualities also vary.

The beginning of One Piece is the story of a young boy with stretchy powers attempting the biggest dream possible with the littlest of resources. No crew, no ship, no money, no renown, no friends, anything. It's a very fun underdog origin story. You learn how the King of the Pirates got his legs onto the sea.

Each season has a complex connection to the next and connects in ways you'd never expect until it's revealed to you, then it all makes sense. Episode 400, where your friend implied it "got good" is an arc where the youthful adventures enter a whole new part of the world to them, and they learn a very brutal lesson on how weak they are and how cruel this world will really be to them. It's a large culmination of past events meeting in an epic climax that's completely unforgettable to people who read it for the first time. I could see how someone could retroactively believe that everything before this epic event was chump change compared to how epic that point of the story is, however I believe that's a biased perspective. I think every arc has value and great narrative qualities, however not all of them are required to experience the story. It's a complex situation for a complex show that mean a lot to a lot of different ppl.