For serious advanced students (Carly, Saiya, I'm looking in your direction) DS KanKen is one of the best purchases I've made in a long time.
I have a lot of ordered memorization down but it doesn't always mean my stroke order, among other things, is dead on, nor that I can produce kanji when asked either on the spot or out of memorized order.
That's why this software is gold.
If you're a new learner it is worthless and I would completely avoid it but this little dream is fantastic for anyone that has at least 500 or so down pat. An EXCELLENT way to practice and rate yourself.
Actually I wish the grading was a little more specific (ie. track what you do/don't need work on) and the games were level directed as opposed to overall (ie. with the taxi game I can get about 5 answers out of 20 right before it starts spouting 'WHAT THE HELL IS THAT MESS' Kanji, but otherwise it's a GREAT exercise tool.
Lucky for me, I'll be in Japan in a week and a half, and will be able to pick this up then. :)