I don't even remember what life was like without one piece, and I can't imagine what I would do if it doesn't exist (which will be a significant problem in 5 years). That's how good this series is; it has the power to grip you and never let go, because the story is so well told that it makes you desperate to see what's going to happen.

Pros:

The artwork is incredible, and the designs of everything are extremely divers; from islands in bubbles under the ocean, to raging islands that are bombarded by lightning 24/7, to cloud cities in the sky, One Piece feels like a wonderland (filled with the stinky sweat of pirates and the harshness of monarchal governments).

The story itself may very well be the greatest ever told, as the plot is told in a way that twists planned out, 5, or maybe even 10, years prior unfold often and constantly shock you as you read on the toilet (or bidet, if that's what you prefer).

Part of the stories greatness involves the depth of the characters; one piece feels like a living, breathing universe where everything makes sense, and there's very few inconsistency, which is impressive given that Oda has to actively keep track of hundreds of characters that are playing a role within the manga.

Cons:

One of a select few series that I won't place any cons. I guess a single, glaring con could be Oda's perversion, but if you don't care about author's personal lives, and don't plan on reading question corners, then you'll be all set (living pervert free for the rest of your days).