One Piece is 10,000 times better then Naruto and Bleach combined. Yes, it does start slow, but I have yet to find a show with as much heart as One Piece has. I defy anyone to not shed a tear at this part in the series:
(that pic is actually from the 8th movie that retells the arc, but I could not find a pic from the series)
Great characters, great humor, great action, great music, and great voice work as well.... It really does have everything. The 400 episodes may look a little daunting at first, but each story arc is really pretty self contained and usually last around 20-40 episodes so it is really more like each arc is a new series almost just with the same characters.