I own five or six racing video games.
I have owned a few more that I have since traded in.

Mario Kart Wii is the best racing video game I have ever played.

Sure, it doesn't have realistic car models, realistic damage, realistic race tracks or pit crews.
What Mario Kart Wii does have is tons of different vehicles, a dozen weapons/gadgets, great and varied race circuits and buckets and buckets of fun.

The game can be played single player off-line or on-line in either Race or Battle Mode.
You can race against your own best times, your own ghost laps, compete to win all of the circuits Grand Prix style or just race for the sake of racing. Battle mode let you zoom around with you teammates in a rush to collect coins faster than the other team or administer damage to the opposing team.

Race and Battle long enough and well enough and you will eventually unlock a lot of things, including: new characters, new vehicles, new events, the ability to use your own Mii in the game, new costumes and more.

On-line, the game shines.
In Race Mode, you are pitted against anywhere from 1 to 11 other opponents on a race track that is voted on by all racers but chosen randomly by the Wii. Races last no more that 4-5 minutes at most and are lag-free. Battle Mode pits your team against another team in a scramble to collect coins and whack around the other team before time runs out. Battles last no more than 10 minutes.

Replayability?
Is "Infinity" a good number?

Once you start playing Mario Kart Wii, you won't stop.

You will take your lumps sometimes, but the races/battles always equal out and are always, always fun.