CST has Rohacell 51 down to 1mm (.04in.) which I think a lot of guys use for a substitute for balsa or Herex sheets. [link=http://www.cstsales.com/Rohacell_51_IG.html]CST Rohacell[/link] A lot of your European high end sailplanes and F5D composite racers are constructed with this stuff.
As for experience with it, it is on my "to do" list.
