I just bought Mario Kart 8 today, so my opinion on it is still forming... but right now I'm giving it an 8 out of 10. There are a lot of little things that are bugging me about this game, like cuts to the roster (no Dry Bones), too many Baby characters, the split screen is vertical instead of horizontal, Battle Mode isn't as fun as Mario Kart 64, the anti-gravity isn't that much fun, etc...
I have already tried multi-player with a friend but I haven't tried out online multi-player. I'm hoping I can revise it to a 9/10 in a few days after I've had more time to play it.