This issue of race and gender in the media is very complex. It's not as simple as "Women can kick ass as well as the men". You can have a female character who is physically the strongest in the series and still have it be a bad representation.
For a Shonen manga, the female roles in One Piece are above average. Oda does many things right, but improvements can be made. You won't find many stories, even in Western media, that write women well.
I think what really helps is that One Piece has little to no romance. The one thing that bothers me about female characters more than anything, is the girl who's sole goal in life is to support her lover and her only ambition is to get married. Or the girl who's entire life revolves around making a man happy. (See manic pixi dream girl) Nami and Robin have their own goals in life and they are not in the manga to fawn over Luffy or some other male character. They are also very intelligent and vital to the crew. We can argue that Straw Hat crew wouldn't have gotten far on the Grand Line with out Nami's navigation. It's also nice that they are shown to be intelligent characters and we were not simply told.
It's nice that they also get their own fights and aren't simply just healers or on the sidelines. Though it is very disappointing that Nami seems to need rescuing at least once an arc. At least Oda tends to subvert it a bit.
While Nami is a bit cliche, I find Robin to be a unique female character. In anime, women tend to come in only a few varieties. I haven't seen many if any like Robin. It's also nice that she's over 25, (now 30) and can still be seen as sexy.
Vivi, while being a princess, it simply wasn't just a pretty title. She was involved in politics and cared more about her country than anything. And she wasn't betrothed to anyone. She also took out the seven pair very easily.
I do like that unnattractive women are included and placed in powerful roles. Kureha was one of my favorite side characters. Lola was shown to be brave on Thriller Bark, and Kokoro saved the crew from drowning. (Though Sanji's reaction to her later at Sabody wasn't very cool. Seriously, the woman saved your life!)
The show passes the bechdel test which is great and it's wonderful to see that Nami and Robin have a close friendship. (Friend ships between women tend to be rare outside of chick flicks)
Then there are a few problems. Oda is guilty of the smurfette principle which is just another form of tokenism. (There tends to be one woman per organization) I also don't like the idea that it's ok to his ugly women, but not pretty women. This almost seems to apply that a women's worth is based on her looks.
Also, and this goes for any anime and manga. Peeping in on someone in the bath or shower is NOT funny. It's rude, disrespectful and frankly, abusive.
Boa's crush on Luffy was also ridiculous. She goes from a strong will woman into a sickly weepy love sick person because a man showed up in her life. It's great that she is strong and can take care of her self, and it's fine if she falls in love with him, but her reaction around Luffy was not ok.
As for the fanservice, would everyone here have not watched One Piece if Nami and Robin had small breasts and we never even saw them in skimpy clothes? Would you like the characters any less? If Zoro was a woman, nothing at all changed about the character in regards to strength or personality, would One Piece be a worse show? Heck if the only change made to Luffy was that he was a she, would One Piece be unappealing? Avatar the Last Airbender was popular among both boys and girls and there was next to no fanservice. Legend of Korra also seems to be doing very well.
And yes Shojo often does have much worse portrayls. Hot Gimmick angered me. I loved Please Save My Earth until the whole fall in love with your rapist happened. However, that doesn't give any other genre the excuse to portray their women a little better.
Though in my opinion, there shouldn't be a distinction between boy and girl shows.
We could also ask how sexuality is portrayed in One Piece. Are the okamas offensive to the LGBT community? Can any LGBT community answer this?
Feminist Frequency and Reel Girls both have good videos that explain race and gender in the media better than I can.