The wikipedia article about One Piece has some interesting statistics. According to it, in 2008 Once Piece became the highest-circulated manga series (whatever that means), achieving a circulation of 5,956,540, according to this page. To compare: Naruto, which had the second place, had a circulation of 4,261,054.
In 2010 it was announced that 260 millions of manga volumes had been sold, and Volume 61 set a record as the highest initial print run of any book in Japan in history, with 3.8 million copies. If you think about it, it's huge. It even beats Harry Potter's sale rates.
It is also mentioned that One Piece is the best-selling series of all time in manga history, exceeding 270 million volumes sold as of February 2012.
Just to compare (although it may not be an exact analogy), here is the list of the most selling book series in history.
You can find some more infomation and numbers in the "reception" part of the article.