Stuck panel screws

Here's the tool I was talking about

http://www.factsfacts.com/MyHomeRepa...pactdriver.jpg
Here is the description as I found it on factsfacts.com's site:
An impact screwdriver is designed to rotate the screwdriver tip when the
handle is hit with a hammer. So, the tip is forced into the screw head by
the hammer blow at the same time the tip is being rotated to loosen the
screw. If you have a motorcyclist friend she may have one. If the screw is
in cast iron, or something else you could break by hitting it, you may not
want to use this.