I've seen a used copy of the game for about 20$ on Wii U (the game's available on PS3/Vita/3DS too), did some people played it ? Is it good ? I've read about the 50 first books of One Piece, I liked them a lot.

It's OK.
If you have a Wii U Unlimited Cruise is better.

Ya the past unlimited games were better.

Thursday it will be on sale on eShop in Europe, if you do not mind digital downloads.

Wii U - ONE PIECE Unlimited World Red – €14.00 / £9.99 / CHF 18.00 until 12/11/2015.

3DS - ONE PIECE Unlimited World Red – €9.99 / £6.99 / CHF 9.49  until 12/11/2015

3DS - One Piece Romance Dawn – €9.89 / £9.39 / CHF 10.80  until 12/11/2015

http://nintendoeverything.com/nintendo-download-102915-europe/

The game is almost 11gb on eShop...I would likely pick the box version for a few more euros instead.

According to eShop it is even bigger: 15,2 GB

I have the game on Vita. The game is ok.
It is something for One Piece fans. If you like One Piece, you will enjoy the game. Otherwise not.

It's definitely the best One Piece Game. I'm actually waiting for something like this for a long time cause I don't like the previous ones gameplay which are like Dynasty Warriors. They finally made something like this so I could battle with friends. Haven't played a one piece game since PS2. I just had Naruto and J-Star Victory on my PS3.

I wouldn't recommend it. Only because if you say you read only the first 50 or so books, then your still in part one of the series. This game has a lot of things from part 2

I liked it even though I have never watched a One Piece episode ever.