The game looks so fun and I've played other Mario Karts. In your opinion, do you think it's worth to buy the system, just for the game?
I would have to probably say yes especially if you are a racing afficionado. Mario Kart 8 is a fun, challenging and exciting racing game. It's a great 1-player game and a great two player game. You would find it hard to switch off your Wii U if you play this game. However, if you hate racing games, then I think you should not purchase the Wii U just to play it. There are lots of other great games on the Wii U that you could play aside from Mario Kart.
Yes I will tell you to be honest that I think that Mario Karts right now is the best game that you can buy on the console. Right now it is also the best selling game on the console so that alone should tell you that it is definitely worth getting if you really want to have some fun on the WII U.
Mario Kart is incredibly fun, although I would look and see if there are any other games you'd be willing to play. It's hard to justify buying a hundred dollar system just to play one game.