This is a hunk of plastic that holds your 3DS. It mimics a steering wheel, just like the steering wheel accessory for the Wii. At first glance it seems kind of silly. Certainly, using this with Mario Kart 3DS is the obvious use, but as others have pointed out - it is a good alternative to holding the 3DS with other games - particularly if you have big hands. And the price is certainly right.
The minor issues are that shoulder button control is more mushy - they still work fine, but it doesn't have quite the same positive feel. The other issue is that it is big. This is simply the way it is, so when traveling you won't be able to put it in your happy little 3DS case. But again - it's a hunk of plastic that can stand abuse.
Will it make you a better game player? Perhaps not. Does it make the 3DS more comfortable to hold for marathon gaming sessions? Yes. Is it a reasonable price? Yes.
The other option that may be worth considering, if you want a bit more more bulk on your 3DS, is Nintendo's Circle Pad Pro accessory. It is more expensive - and is not guaranteed to work with all games out there - the game expressly has to have support for it. But it's extra size may make it more comfortable to hold. And of course, there are other grip add-ons for the 3DS as well.
It is kind of funny to think that Nintendo's industrial designers worked hard to make the 3DS a reasonable size (which explains it's somewhat mediocre batter life) - and now we have several accessories that make it bulkier - and actually makes it easier to hold for long sessions.