It is fluid, colorful, playability is perfect, the difficulty is always dosed with 3 modes (50/100/150 cc) that allow players of all levels to find IA their height. For those who have seen the turn Mario Kart arcade version, here it is made on the same level of fluidity and sensations.
Big improvement over the Wii version, for those who play in split screen: the game 2 is as fluid as in solo (Wii, we had a quite noticeable loss of fluidity). A 4 is slightly less fluid, but it's still very good.
The online mode allows you to compete against players from around the world, again with a welcome improvement: you can play two on the same console, while facing online players.
Another important development with Mario Kart 8, Nintendo goes to DLC. A pack of 8 new tracks and 3 pilots was released in late 2014, and one released in May. A very good way to increase the lifespan of a game already very replayable, and significant since the price of additional content packs is very reasonable (8 pack or 12 for 2).
In short, a perfect game, that appeals to players of all ages from 4-5 years, who can almost alone justify buying a console Wii U.