I don't have a 3DS but as far as I am aware if you attach this to your 3DS then you can't use the circle pad add-on right? Also, to attach this don't you need to use screws and stuff?
I am not bashing the item, I do really like it and if I had a 3DS I would want it, but the fact that it removes the use of the add-on makes me question it's long term usefulness. But hey I could be totally wrong about it.