> Is anyone using this sucessfully? What should a setup look like.

  I've always used union mounts for builds.  Works great.  (Well, there is
reportedly a bug that can bite if you modify the lower layer, but I don't do
that, and it has always worked very well.)  It also makes it easy to
automatically apply local patches without disturbing the source tree.  And,
it makes it easy to clean up afterward: just unmount and delete everything
in what was the upper layer.