They were being discussed on another forum that I visit, and a guy that owns one says that although it's a pretty nifty feature, you do pay through the nose for it.
As mentioned, it sticks the etching in the unused part of the CD and effectively fills it up.
You also need to have CDs with a dark blue dye in them in order to see the etch properly. If you use the cheap CDs, it can be very difficult to see the etch unless the light hits it at just the right angle.