One Piece takes a while to get going and can be a rather difficult franchise to get into, especially with its raw length. The series is largely dedicated to introducing protagonists early on whom only improve as it goes on, starting out fairly lackluster. Luffy and Zoro, the main protagonists, are generic and largely exist just as generic protagonists to go through the arcs and defeat the villains. The other 7 crew members all get pretty heavy development with plenty of likable qualities, though, so it's not a slog aside from the start. The series really begins with the Arlong arc a good 75 chapters in.

The series should largely be read for the big villain arcs, as they are the highlight of the series. While the minions of the villains aren't used much, this is largely done to glorify the current arc's villain above all others and is done to pretty great effect. Some arcs, most notably Rob Lucci and Crocodile, tend to drag a bit, but these are few and far between.

The best villains are those whom have dedicated arcs to themselves and are defeated, but the series has massive amounts of power creep with characters getting stronger and stronger. This alone wouldn't be that bad of a problem, as it is expected in most any shonen, but the characters who are built up as invincible threats are background non characters such as Shanks and Mihawke. Constantly glorifying characters such as these who do absolutely nothing over villains who had great arcs gets very tiresome very quickly, and the author constantly dangles them in front of the reader's face with promise to actually use them then never actually does.

In addition, the author has an obsession with not allowing any characters to die, even retroactively resurrecting the most minor of characters. This is almost always a bad thing for villains, as instead of dying they're retroactively humiliated when they show up much later in the series as weaklings. The one exception to this is the Impel Down arc, which is a prison break arc involving a lot of the old villains.

The manga is at its best when focusing on just one big villain at a time in an arc for the most part, and these segments are plenty worth reading the manga for. The only big continuity comes from the Impel Down and Marineford arcs, which brings everything to a pretty satisfying climax and may as well be the end of the manga, as only a few relatively episodic arcs have been written after it.

One Piece does a great job of world building as it goes along, introducing dozens of characters and locations it goes out of its way to develop and make memorable. The manga begins to go a bit overboard with the quantity of characters it produces, as the author is incredibly ambitious with his assumption that he'll get around to using them all. The flood of characters can make it difficult to get some of the earlier styled arcs focusing on a single villain, especially after Marineford.

As messy as One Piece can be, at its core it is a very simple shonen that does little to subvert the genre with good climaxes and fights, which is great if that's what you're looking for. Later on, some of the villains decay and the protagonists become too powerful for the villains to put up much fight, but the best moments of the manga are already past you at that point.