I have been watching One Piece the anime for a while now. True as what Endric has said there has been close to zero fill ins as compared to other Anime series.
Endric explained that One Piece was created using a different concept as compared to conventional animations. Namely it is an objected oriented structural approach whereby instead of one author having to draw up the whole series himself, he engages instead a team of comic artist each of them concentrating on just one character.
This approach results in a very diverse variety of content as well as increased mulit facet building to the characters within the stories itself. making the anime series richer than most. All these on top of the fast turn around time in churning out interesting stories. Comic artist also could leverage on each other to avoid the normal writers block, since they could draw inspiration from the ideas of their team mates.
All in all, I would give it a two thumbs up.