I bought one. For normal work, i just use the D200's pre-set white-balance, e.g. flash. It is spot-on outside with sport. Last year I also visited a bird hide with tainted glass, and then this gadget was handy to get the blue cast from the glass pretty much away. I also have this grey card, http://www.rawworkflow.com/products/whibal/index.html but I somehow prefer the expodisk.
I have one and I use it all the time indoors. On my D200 the auto white balance works great outdoors. Indoors with a mixture of outside light and indoor light its not great but good. With the Expodisc and manually setting the white balance in this situation its great every time! I only use JPEG(for now) so getting the WB spot on in camera is critical. I think it is worth the money and recommend it.
hope this helps!