Articles like this make me feel like an ignorant baboon. I've never even heard of One Piece.
One Piece is a Manga created by Eichiro Oda awhile back. It's the tale of the Straw Hat Pirates, searching the world for the greatest treasure ever, the one piece.
There are numerous characters with super powers of various sorts (you can get those powers by eating something that's known as a devil fruit -- the drawback being that you lose the ability to swim), some good, many bad.
I read a fair amount awhile back. It's pretty basic shonen -- lots of fights, pretty girls, friendship and admiration -- perfect for the 12-18 crowd. That being said, Oda-Sensei has a dynamic line that goes well with the cartoonish art style -- reminds me a little of Carl Barks -- plus he writes characters that aren't just one-dimensional tropes. I gave up after a bit because I felt like it was getting repetitive, but I can understand it's popularity.
I've never read the books or watched the show, but I have friends who are HUGE fans of the anime. One of them said you could open of a store that shelved just the manga, and it would be full.
The guy who used to host the bar trivia I went to, his wife was the voice actor of the main character of the show.