I don't think I need to explain why One Piece is fantastic and a narrative masterpiece. All of the other reviews have said what needs to be said in a much better manner than I ever could. So, instead of doing the easy job and telling you why One Piece is so great, I'm going to detail my ONLY major complaint: the fan-service/depiction of women.

This "review" will have minor spoilers.

The fan-service before chapter 600 was bearable. It crossed my boundaries maybe once or twice in the Thriller Bark Arc, but, otherwise, I didn't mind. But, my goodness, once chapter 600 hits and we get post-timeskip Sanji, I'm just barely able to like his character and Oda's depiction of women. Thankfully, after Fishman Island Arc, Sanji becomes his better self, but I still am not a fan of how Sanji is written, and how Nami and Robin have basically become outlets for Oda to draw over-sized breasts. It's unfortunate because none of the female cast is written poorly, per say, but it's obvious that their designs only really serve to get the bodies of little boys to pump out hormones faster than Luffy's Jet Gatling. I really hope the fan-service gets toned down later, as I'm only on the Punk Hazard Arc as I write this.
I'm sure that, for a lot of people, this is not an issue. And I think it's important to note that the fan-service doesn't degrade any of One Piece's other outstanding elements. But, I'll be honest, it's starting to drive me crazy, and, if it continues, it will seriously lower my enjoyment of this otherwise amazing manga.

TL;DR fan-service bad, but not Fairy Tail bad