Well, I guess the guy's awesome, then. I mean, he likes One Piece. He's bound to be a rad guy. :p
And it's actually great. Might seem childish at the beginning but it's not. It is, simply put, one of the best adventure stories I've ever read in my life. People are turned off by the length of the series and the fact that the art style is so wacky. But those who get over those "shortcomings", as they'd like to call them when they haven't experienced the epicness of the series, enjoy it a lot.
You ought to try it out sometime. Those who I've recommended it to and have given it an actual chance loved it.