What's good about One Piece? (<- honest question) It just seems too cartoony to me.
Well, I'll tell you what I like about it.
For a while, I was bringing 2 volumes to work everyday. I would read through about half of it in 15 minutes on the bus, finish it on my short lunch break, and start the next one while waiting for the bus home after work. The reason I mention this is because the narrative is SO engaging and can be unfolded really quickly.
Everything about One Piece is so incredibly gratuitous and rewarding. It reads the way an epic RPG might play: adding party members to Luffy's crew with specific abilities (A lanky chef with brutal kicks, a raindeer with the ability to change into various super strong humanoid forms, a master swordsman/bounty hunter who fights in a 3 sword style...); there's constantly the mention of a seemingly impossible to beat foe/organization, but by the time Luffy and his crew encounter them, there has already been made mention of a stronger, even MORE impossible to beat foe; pretty much everyone in the world wants to be the best at SOMETHING (best swordsman, best pirate, w/e).
The drawing is phenomenal, also. I'm an artist (Smash is just my day job, LOL), and every one of the comics I mentioned above is not only amazing in terms of narrative, they're generally breathtakingly beautiful. One Piece is gratuitous even in it's drawing style.
Berserk, for instance, is beautiful in it's brutality. The artist uses some really strange layering of lines/hatching, benday dots, graphite splatter, and ink to create really visceral, bloody spreads. Single frames could be paintings. Unlike Berserk, One Piece is SO masterful in it's rendering and it's cartooniness. I love that about it. It's really stretchy, always in motion, and crisp.
I could go on, but I'm just going to have to tell you to start reading/looking at the series. Start at the beginning if you can, and you'll loose weeks of your life to it's all-engulfing narrative.