First season starts with mediocre visuals and filler episodes, correct.
Second half of the season has one of the most fluid and dynamic animations I have seen in the western media, combined with an appealing style.
The story picks up the pace as well, and peaks with a bittersweet ending and a really charismatic villain.
After that it's a steady decline in quality. Second season is meh, third is bad.
Animation is still decent, but it looks a lot more generic weeb.
And the story turns into shit, full of holes and just boring.