I just spent app. 800 DDk on an 77mm ExpoDisc for my Nikon D200. The ExpoDisc ensures a more accurate white balance, ses the pictures below:
The first picture is without the use of ExpoDisc and on the second one the ExpoDisc has been used to set the white balance. The best of it all – it works perfect in real life!
A real review can be found at kenrockwell.com