This is the biggest anime I'm ever going to review, so hold tight for One Piece.

Okay quick run of the plot; Monkey D Luffy wants to be Pirate King so he gathers a crew and goes in search of the One Piece and is still going over 500 episodes later, there's your plot now lets review it.
After all these years One Piece is still just as good as when it first started and unlike other Shonen Jump shows doesn't fall into an endless cycle of trying to beef up it's story to obscure levels.
Allow me to expand (Joke not intentional)
You see a long series like One Piece is always going to have obvious strengths such as good action and colorful characters, after all one of it's biggest selling points is vivid color themes and creative design, but with such a long time spent on setting up it's characters, it's beginnings couldn't match how well Naruto & Bleach did in the same time period as they didn't spend so long introducing everything and were already doing defining moments in their respective shows such as the Ichigo-Byakuya battle and the Rock Lee-Gaara battle, two of anime's best fight scenes.
But once One Piece got going, it was creating it's own defining moments while the other two were on the decline; even now when Bleach is making it up as it goes along and Naruto is pretty much now called Uchiha, One Piece is still the same since it began in 1997 and that's the big secret to it's success, nothing has changed for the Strawhat Crew but the World around them and they all adapt without losing what made them in the first place.
Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky & Brook have gotten better at what they do but are still the same inside. They don't need impossible jutsu or an ass-pull power up, they just continue doing what they do best. Fairy Tail is the same, in some respects Dragonballz is the same and Pokemon is the same. Status Quo is necessary for a long show's survival and One Piece always nail it, that's why it can still keep going and will probably end another ten years from now.
Final Verdict: I understand that a large long running show is very off putting to most but I still think One Piece deserves a lot of respect regardless. If you are able to devote the time to watching it then give it a try, you won't be disappointed.