First Impressions:
Firstly, the game is beautiful, it's very Mario Kart-ish.
Secondly, the maps are amazing, none of the new maps are overtly stupid in design, though Rainbow Road is pretty underwhelming to be honest, unlike past Mario Kart games. Sure it's pretty, but it just have that Rainbow Road feeling, not enough unique obstacles that make it stand out, in fact, it's pretty bland and straightforward compared to the rest of the tracks in the game. As for the Retro Cups, all terrible choices for tracks, the only decent choices are Tick-Tock Clock and the N64 Rainbow Road (which they used Double Dash RR music, which was a nice touch), though the Mario Kart 8 touches they bring to em, do improve them somewhat such as being able to underwater in Sherbert Land without just dying.
Anti-grav is a nice touch, strange how you get a boost from bumping into people. Can be disorienting at times though. The gliding through air and underwater stuff is pretty nice, though gimmicky.
Character choice is pretty nice, though Metal Peach (and Luigi) are terrifying. I also feel somewhat they just added in the Koopalings to just make the roster seem bigger. The addition of ATV's to the mix of vehicles is interesting, I wonder if they will expand upon them next game, considering there's only two of em in this game, probably as an experiment.
Replay sytem is hilarious, though I wish you could record slow down and fast forwards and stuff, maybe next game.
Aw man, I have an awesome shot of Koopa Troopa getting hit by a Blue Shell. Just after hitting someone, he's all happy, then he looks up and sees the Blue Shell and his eyes follow it as it's coming down and it cuts to black just as it hits the ground. It was hilarious.
So far it's good, but not as good as or better than Double Dash....yet. Nothing has yet to beat that one.