This is a great manga. I just want to get that out there first. Before I read One Piece for myself I heard some people saying it was a Dragon Ball ripoff and while I can see where they're coming from sorta (Luffy does remind me of younger Goku quite a lot) One Piece is by far its own thing, and a pretty amazing experience.
I won't lie though, the series certainly has problems. For one, (and some people willl probably hate me saying this) but I think the series is just too gosh darn long. I know it's a shounen and it's popular so of course they're gonna stretch it out, but at some point I think it just gets kind of crazy. One Piece has hundreds of chapters and anime episodes, and like a dozen movies. While it does make the series epic in scale, it also means that some plotlines just take forever. That and One Piece also has lots of characters. Tons in fact, and they don't stop being important to the plot when they leave so sometimes you have a break of 300+ chapters before a character returns. This is fine for the most part because it helps build the world but at the same time it's clear this story will be going on for a couple hundred more chapters at least. This means they'll be adding in more characters and it's already making my head spin. Heck there's 10 main characters in the Straw Hat crew, and in some of the later chapters the story starts to buckle a bit trying to keep the reader informed what's going on with everyone. Also while there's all sorts of varied character designs throughout the series, after a certain point nearly every woman becomes the same basic template (enormous tits, tiny waist) which isn't a huge deal because hey this is for teenage boys but does make me wish he tried harder with the female character designs.
Mostly this is just nitpicking though. I mean the story is over 800 chapters by now, if you see that and you're still interested you're probably going to be cool with a little complication. Since I've been following the story for the past five years or so I don't think much about the length at this point but it will be one of the biggest hurdles for someone coming into this series fresh. If you're willing to put in the time however you'll be greatly rewarded, One Piece is just a blast to read. There's a lot of incredibly creative powers, awesome fights and some fantastic moments of friendship. A surprising amount of emotions here as well, at a couple of points during the story I shed some tears or smiled in pure joy. I think this manga is primarily about the joy of life, about friendship and love and fighting for what you want.
Then there's the setting for the world, which really lends the series a wonderful sense of mystery and adventure. The island setting lets each new location be as crazy and varied as Oda wants, and since they reveal bits of the plot over the course of the story you always want to read more to find out if they talk more about one of your favorite plotlines. I really can't wait to see the end one day, just so I can see where Luffy's story concludes.
I really recommend this story. Sure there are some parts that aren't as good, but this is one of the most popular mangas ever for a reason. The story is fun, the characters are memorable and you feel like these people are on a real adventure.
P.S. - The best shounen is Fullmetal Alchemist and the second best is My Hero Academia in case you're curious.