I purchased this for Christmas for my 5 year old. We haven't tried it yet, but it looked so cool, I couldn't resist. Family Fun rated it pretty high in their annual best new toys issue, which is where I first heard about it.
My DD first saw it in Good Housekeeping a few months ago. I figure it probably works well or it wouldn't be in that magasine since they chick everything out so carefully.
We couldn't find a single black crayon in the entire house last week when DD was working on a project for school??? I dug through all the bags and boxes of old, broken crayons but the best I could do was dark purple!