One Piece is a beautiful, beautiful manga; however, many people overlook it because it stands in the "Big Three" manga, which are highly criticized.
I admit, I did NOT like One Piece at first, and it bored me to tears at points. That was about 4 years ago, when I still did not have a full grasp on the struggles life had to offer me. I resumed OP around 2 months ago, and I saw it in a very different light.
What I saw once as a silly pirate story became a ongoing, complex story about finding one's passion, friendship (or in this case, "Nakama"), adventure, and overall, growing up (this is especially evident in the later chapters).

Story (10/10):
You might not see it this way if you are still young, or rather, in the "I-don't-know-where-I-belong age group" (around 12-14). If young enough, this is an comical adventure story. If old enough, the plotline is amazing and the characters -- just right. I started OP when I was 12, but only after 3 years in high school did I notice the similarities in the challenges faced in this manga and real life. Whether it be a quest for an "impossible" dream or just pure acceptance, One Piece touched many edgy topics in a way that was both serious and comical.

Art (8/10):
The art is simple and unique when it comes to characters, but the background is GORGEOUS. So gorgeous that it draws away sometimes from the actual action. For example, if two characters were fighting in a room, the detailed curtain drapes might confuse you. But, overall, the feeling is not lost. The characters' features were hard to get used to at first, but then I realized Oda was a genius for making them this way: the features were perfect for both extremes of the emotional spectrum. Comical because they were flexible (especially Luffy, ha) and stoic because they were firm. One flaw I see, though, is the women are anatomically disproportionate, even for manga, being incredibly skinny with huge breasts. Huge.

Character (10/10):
Oh my goodness, this makes up the whole show. No joke. With each addition to Luffy's crew, you feel a certain connection, all because of two major points: bizarre characteristics and awesome background stories. My goodness, the BACKGROUND STORIES. I wish I could tell you more without spoiling, but I can't! They're just beautiful! However, characteristics you may see that no other character possess are Zoro's complete (COMPLETE, COMPLETE) lack of direction (COMPLETE), Chopper's insulting gratitude, and much much more.

Enjoyment (9/10):
One Piece keeps you on your toes because you don't know what's going to happen next. Of course, an adventure, but where? These places are not for ordinary people with ordinary actions. The rest of the enjoyment comes from many of the qualities above (character, especially).

Overall (9/10):
I love this manga so much. So much. It makes me feel better when I'm down, and laugh even through the saddest times. It makes me cry with the characters and yell at the pages sometimes. Just don't judge it by its first few chapters, because it WILL get better and mature along with you.