My friend bought Mario Kart and we played it on Friday and I was a little dissapointed. I loved all the other Mario Karts but its just lacking something. The multiplayer of course is the major reason to buy this game but if you plan on just playing it solo most of the time I suggest you buy something else. I beat 50cc,100cc, and 150cc in about 3 hours and then there were just the time trials. 
I guess with all the hype this game got I was expecting something awesome but there were just 12 race tracks and they are all the same in all three cc's except for a barricade down here and there. We also played the Multiplayer and I found it lacking. 3 modes which are ballon burst,steal sunshine, and b-bombs. Balloon burst is just like the old Kart and has 4 areas, steal sunshine you get a star and have to last with it for a given time but if you are hit you lose it and other player gets it and so on with 4 areas, and the b-bomb you try to hit each other with b-bombs for 3 hits with 4 areas also. I just found it lacking. :/
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Edit: Forgot to mention that it was just 2 players but the Battle modes were hectic I can't even imagine it with 4 karts and 8 people playing.