Have always run the Mr O inserts since they became available and find them consistently good and don't seem to disintegrate like some others.
I always run the extra wide one in the rear on any tire other than a schumacher silver or Dboot when the chubby insert seems to work best. Then a firm front for both 4wd and 2wd.
Website is
http://www.mroracing.com/foam-inserts.html