My two cents on it - I'm pretty crazy about it, and so's my wife. Coconut Mall is my complete favorite right now, but DK Summit and the partially submerged track (in the same circuit) are quickly becoming two of my favorites.
One highlight about the game so far is how they set up online play - I think it's rather neat that everyone's Mii pops up at their location on the map before gameplay starts.
Drifiting - if this was present in the other Mario racing games, I must admit I never caught on to it...but this one made sense to me. Why that is, I'm not real sure. I love doing the tricks off the jump ramps as well.
Bikes are a neat addition (wife, again, loves them), though not as hard to get used to as I'd heard. As far as the Wii Wheel goes, I've never been able to use the Wiimote on it's side (as is "Cars") as a steering device very well, so I found the wheel to be a delight - I finally feel like I've got control over my racer. There are some cool unlockable vehicles as well, but I'll leave that there to avoid spoilers.
Is it the rush that Mario Galaxy or Smash Brothers was? Hmmmm...probably not, but it's certainly a 9 on my fun meter....and once again, replay value is unreal. It's a nice addition to the series.