Pros: Easy to learn, very fun, a MUST for the DS, ONLINE PLAY!!!
Cons: Hard to add online friends, no online blacklist
The bottom line: A must-have for DS owners, the best multiplayer ability we've seen from Nintendo thus far. A welcome addition to the great Mario Kart series.
Introduction
What is Mario Kart? Well, if you're reading this review then you've probably at least heard of it from your kids...but in short, it's a kart-racing game by Nintendo. The game allows you to race your favourite Nintendo characters against each other, and use nifty little attacks in the process. This game represents a step forward for the series, and a leap forward for the Nintendo DS system.

The game
This game has a pretty good balance of being conservative and exploring new ground. Like previous games in the series, you have about 8 grand prix series comprising of 4 tracks each. As you progress through the game, you can unlock different grand prix series.

In this edition of Mario Kart, they've added a whole new mode called 'mission' mode. In this mode, you must complete a series of objectives before reaching the time limit. At the end of each group of missions, there is a boss stage. Examples of missions are collecting coins as you race and passing through numbered gates in the correct order.

There are a lot more characters to choose from in this game. In some previous games, the character didn't make a whole lot of difference in the actual game-play besides having a different head sticking out of the kart. This time around, each character not only has a very distinctive speed, acceleration, weight, etc., but a brand new variable has been added. In this game, there's an 'items' score, which dictates how good the items you get will be. If you've never played Mario Kart before, then items are basically different attacks that you can launch against other players (although there are other items, such as boost, that do not harm other players).

Like previous Mario Kart games, the handling is very easy to learn. If you've ever played a Rally racing game...this game is nothing like it. However, that doesn't mean that you can't master this game. As you progress through the game, you'll learn new tricks to give you an extra edge over the competition. Of course, if you've played the game before, most of the tricks still apply in this game.

Like a lot of Nintendo games, sound and graphical quality aren't big concerns, but if you're wondering, the game looks and sounds great. The backgrounds, characters, and tracks are all very pretty and appealing.

Multi-player
First, I'll start off with the basic, no-wifi multi-player functionality. In this version, you can pit up to 8 players against each other simultaneously using only one card! This was one beef I had with the GBA that I'm really glad they sorted out with the DS. If you have kids, then you only need one card for them to all be able to play each other. Of course, they'll still whine about how they need their own (granted, it'll let 'em unlock characters for themselves), but you can just ignore them when they do that ;)
Now for the fun part!!! This is Nintendo's first game using the new Nintendo WiFi technology. If you already have a wireless router, then getting your game to work may or may not be easy, depending on your router. It worked fine with mine, but some of my friends have had problems. You can find a list of supported routers here: http://www.nintendowifi.com:80/customersupport/supportedRouters.do. There aren't too many listed, so they might still be testing some. If you don't have a router, then you'll need to either purchase a separate dongle to connect to your computer, or you'll have to find a hotspot here: http://www.nintendowifi.com:80/hotspots/searchHotSpot.do.
I'm pretty sure that this is Nintendo's first attempt at online play, so understandably, there are a few problems. But still, they really should have looked at how companies like Microsoft handle their online play before jumping into it. The main problem is the lack of a blacklist of people who you know bother you. Another problem is that you need a person's friend code in order to add them to your friends list. You don't get to see people's friend codes when you play against them- only their nicknames. Talking about the friends list, it can only hold 36!
That being said, I haven't noticed any lag when playing online, which is impressive considering the fact that everything is wireless! When playing, you get to vote on the circuit between each race, which is a much more fair way than having one 'host'.

Final notes
This game is definitely one of the best games to come out on the DS yet. Whether games like Metroid Prime and Animal Crossing DS will top it is yet to be known, but for now, this is a must for the DS, and a great value for any DS owner. Even if you don't usually like racing games, this game is so approachable and fun that it's surely worth a try.
There are very few places where this game fails to impress me, and Nintendo has done a great job with their online service, especially since this is their first venture in that area...on any system! It's great that Nintendo has a network of hotspots you can connect to, as this offers an option for those of us who don't have a wireless router, and don't want to put the money down for the wireless dongle.

My overall rating for this game is 9/10. It was an AWESOME game, and it's worth buying for any Nintendo fan. My only problems with this game were very minor, so if you're wondering why I didn't give this game a 10/10, then...it's hard to explain, but I hope you understand that I don't like giving 10/10 scores out too often, and while it was an excellent game, it just didn't give me that extra 'something' needed for this to warrant a 10/10. Nonetheless, this is CERTAINLY one of the best games for the DS, and a must-have for DS owners.