I have to say, this is probably the best manga I ever read -probably the best manga ever made. It has been publicized as "Dragonball's heir" but, in my opinion, it has so much potential it could even surpass Toriyama's masterpiece. It has all the qualities that define a great manga and, most important af all, it's highly original.
It takes place in a well-detailed -albeit silly- world, where people can obtain super-power eating rare fruits. The main character, Monkey D. Rufy, has eaten one of those fruits that made his body just like rubber. Using his new-found power, he sets forward on a journey to become the most powerful pirate in the world. During his adventures he'll find lots of companions like Zoro the swordsman and Nami the thief, the ones appearing in the firts volume.
One Piece is more like Saiyuki than Dragonball: its main themes are adventure and friendship, not just brawling. Sure, there are fights and they are even cool -given the special fruits and their countless super powers- but they exist as part of the adventure. It's not just saving the world from the enemy-stronger enemy-even stronger one like Dragonball. One Piece will also make you feel real emotions -like Rufy's gratitude and sense of guilt towards Shanks or Zoro's resolution- and sometimes it will even make you cry.
Let me give you an advice: buy this book. Buy the second volume and even the third one -and maybe the fourth one as well. I write from Italy, where One Piece arrived first in July 2001, and right now we are around the 25th volume; One Piece starts slowly but then gradually evolves -both in style and plot- until it becomes, in one word, a masterwork. Believe me.