I am not as against it as other people on here, but you cannot be nearly as creative, and it is not going to be as good as a homebrew. And in reality, a five gallon bucket has just about as much footprint as the Mr. Beer Keg. Obviously it is more vertical, but it takes up about as much floor space.
If you do decide to go that route, a couple of things...
Dont use plain white sugar. Buy some DME and use that instead.
Also, dont use the yeast that comes with it...buy some good fresh yeast.
Lastly, age longer than it says. If I am not mistaken it says two weeks in keg, one week in bottle. I would go with no less than three weeks in the keg and three weeks in the bottle.
If you follow those steps, you can make a decent brew. But I have to ask, if you are buying extract, and buying yeast, and the 5-gallon bucket has the same footprint...why not just homebrew?