Now I'd be willing to assume that most American anime fans already know that One Piece is really popular in Japan, but I'd be willing to bet that they don't know to exactly what degree it is. To put it simply it's the most popular anime in Japan right now. We're nearly talking Dragonball from the 90's level popular. Now I wasn't living in Japan at the time, but from what I've heard One Piece is the closest thing we've seen to Dragonball level popular.
Trying to explain exactly just how big One Piece is, is a bit difficult. I could simply say that literally people of all ages and genders read it which is not that common for manga. You'll see men, women, elementary school students, people in their 40's all reading One Piece. I could also say that One Piece is the major ad sponsor for Axe body spray and Dominos pizza in Japan. You can also see them on razor blades, potato snacks and Pepsi. This is just a guess, but I'm willing to say that there aren't many people 13 and under who are buying razor blades and Axe body spray so clearly they are using it to attract older men. So by looking at all of those products it should become very clear that companies whose products aren't just focused on kids or Otaku want One Piece to be their face. Perhaps you could say it's mainstream like The Simpsons in America.
Currently in a school I work at there is a collection of One Piece figures proudly displayed on the computer desks in the middle of the teachers room. Also in the schools I work at, countless, literally countless students have an endless array of One Piece swag. One Piece is as mainstream as baseball, fried chicken and beds. It's just huge.
One Piece's TV ratings are consistently around 8.0 to 10.0 which is very good especially considering it's Sunday morning time slight. Those ratings are close to what mainstay staples like Doraemon, Crayon Shin-Chan and Detective Conan typically get.
When it comes to One Piece it's a show and manga that's clearly for everyone.