As of April 2012, One Piece is 544 episodes long. Put another way, that is OVER 180 HOURS OF CARTOON PIRATES. In the space of this show, you could watch three or four medium-sized anime and still have room to, you know, go outside or something.
So I can't really recommend this show to a newcomer to anime, or anyone who only has enough time to watch a few episodes a week. If, however, you've already watched a few anime and are hungry for more, or if you're the kind who just likes to latch onto one really good show, or if you have a solid week that you can just throw in the trash, then I can't recommend any show higher.
You'd expect a show of such length to be full of filler episodes and over-extended fight scenes. At the very least, you'd expect it to get repetitive. The fact that all three of these have been (for the most part) avoided is nothing short of amazing. Each arc is unique and perfectly realized in terms of structure and pacing (except maybe Sky Island).
On the more intellectual side of things, this show has an incredible philosophy that's hard to define. Shows like Fullmetal Alchemist and Deathnote are much better at articulating concepts like the fragile boundary of death, the relativity of good and evil, the significance or insignificance of one, etc. etc. But all of these are fully present in One Piece, molded into a carefree / fatalistic worldview that I personally find quite beautiful.
The author chose his setting wisely: the entire world of One Piece is a carefully constructed MacGuffin that serves to cram as many badass characters as possible into a small chain of islands. It's a lot like Justice League, including the guy who can talk to whales.
And like Justice League, One Piece brings home its battle-bacon by stealing combining a large variety of anime fighting cliches and styles. Sometimes these are interesting spins like Zoro's three-sword techniques. Sometimes they are satirical to the point of hilarity (look up 'Tactics 15' on Youtube, even if you're not going to watch the show).
Finally, One Piece avoids the power-gradient that is seen in so many shounen works: the chakra of Naruto, the reiatsu of Bleach, the everything of DBZ, in short the metric unit for measuring a character's strength. While there still is some of this in the Bounty Values - and later on in Haki - One Piece for the most part determines power on a character to character basis. This might seem like it would get confusing, and sometimes it does, but over all it makes each new fight that much more exciting. And don't worry about the characters not being terrifyingly powerful, they still are.
Animation and Sound: 7/10
The animation and sound are amazingly...average. P.S. you better be watching the subbed version.
So why make a show last five hundred episodes? So you can fit in all the characters. One Piece might be simplistic and action-oriented at times, but the main characters are just as eclectic and well-developed as any television entities I've seen. This includes the protagonist, Monkey D. Luffy, who I find gets a lot of flac by people who weren't paying any attention to the things he said. And this is not to mention the ridiculously quantity of minor characters, at least two-thirds of whom you will actually remember afterwards.
This anime is my personal favorite, and the only fault I can possibly find with it is that it's really freaking long.