I liked this show but I found it lacking in energy at times and the actors did well overall but I don't think they picked the absolute best actors.
In the anime series, all the characters, except Zoro, are over-the-top personalities. The acting was more true-to-life which is not what most Anime is. Everyone had a fairly standard every-day enthusiasm. Lufy Is typically much more exitable, Nami is way more angry, Sanji is love-drunk and Usop is far more cowardly.
The most superficial complaint I have is that Lufy has a strange accent. He pronounces a very hard S which is not normally heard in American English or in the dubbed series with American English accents.
It just bugs me because no one else has that accent, including Lufy's grandfather. It ruins the feeling of them being related. It's too obvious that the actors are from different countries.