I would suggest having a case because you'll probably end up throwing your 3DS at a wall.
Music
The music in MK7 is pretty good. It never really gets annoying or repetitive to the point where I feel a need to turn off the sound. Overall, the music is enjoyable.
3D Effects
Absolutely amazing. Almost no ghosting at all. The 3D is strong, but not strong enough to make you eyes hurt (depending on the person of course). The 3D effects are very enjoyable and quite possibly the best 3D of any 3DS game so far.
Gameplay
Gameplay is great. There's more levels than ever. 8 Grand Prix (2 of which have rainbow roads). 4 of these Grand Prix are made of entirely new levels. The other 4 Grand Prix consist of levels from past Mario Kart games. The wide variety of levels never gets boring.
New features such as flying and driving underwater were used just the right amount. There isn't too much or too little of either. Nintendo really did a good job with this part of the game.
Coins you collect in races are used to unlock new car parts. Car customization is great and certainly adds to this game. Getting coins can sometimes be a pain due to losing coins every time you get hit by an item. Also, you can only get a maximum of 10 coins per race and no coins can be kept from Time Trials. No items means no coins, so you'll have to bear the this game's downfall... the items.
Controls
The controls are similar to any other Mario Kart. Nothing new here. You can move, turn, jump, and drift. This game is different from Mario Kart DS and Double Dash in the sense that it does not have snaking. It's good for some and bad for others. The absence of snaking doesn't affect the game other than make the online a bit more fair, but the online has enough problems as is.
Online
This is where every ounce of potential this game had was completely shattered. If you don't have a case for your 3DS, I would strongly suggest you put one on before playing online. Otherwise your 3DS will most likely be shattered from when you throw it at a wall.
The online in MK7 is basically 80% luck, 15% skill, and 5% why is that guy teleporting all over the track?!?
The online has no lag on any players' screens that would affect their gameplay, but it is noticeable in many other ways. Instead of glitching up, other players will appear to drive straight off the track, but then reappear in front of you and continue on their way. Sometimes characters will teleport all over the track. Neither of these affect the gameplay since none of it is actually happening to that person.
Items... here's where the game is almost all luck online. The items are completely ridiculous and unbalanced. People in 1st can still get red shells, people in 2nd can get triple red shells, and from 4th place on you can get pretty much any item. Items such as Fire Balls can cover a huge area at once causing you to spin out 4 or 5 times without being able to move for a long period. Going from 1st place to dead last can easily become a regular thing. Red shells will follow you through air and sometimes randomly miss enemies. Items make the online unbalanced, broken, and almost unbearable to play.
Recommend it?
Overall, I would recommend it. It's always a fun game to mess around in. But don't waste your money if you plan to spend endless hours online.