I have to warn everybody that you won't be able to access the keys on your pen anymore. Simply impossible. But if you're okay with it the hand relaxes a lot more. Actually what I personally always wished for- and somebody seems to listen to my prayers. ;)
The grip can be purchaed via Amazon. If you're not from Germany you'll get the detailled information on the German site anyway :)
*FOLLOW THIS LINK*Wacom Intuos4 Grip Pen - Digitalstiftgriff (Packung mit 2 ), ACK-30002
As I said: 2 grips for 13- 18 euro (depending on the shop)
My rating: Awesome. One of the best inventions I got in my hands.