Egg Chair?

I noticed an advertisement for an "ergonomic egg chair" and decided to take a peek.  It looks like the same sort of idea as is behind those ball chairs, except instead of sitting on a ball, you're sitting on the tip of an egg.  The site that's selling them is http://www.sellcom.com/ergochair.html.  The price listed is $198 with free shipping to the continental US.  Personally, I'm happy to stick with my Herman Miller Aeron, but if anyone has tried an "egg chair" and would like to leave their comments, I'd like to hear some opinions.