Consumer reports did a small study quite a while ago. None of the additives they tested made any differance. But, none of them seemed to do any harm, either.
As anectodal evidence, I once used Restore on a '69 Mustang 351W that had about 150K miles on it. I swear it ran a little better. And I know it burned less
oil. It allowed me to run it for another year before a rebuild. When I removed the pistons I found 2 bent rods, and three broken rings.
There's a lawsuite going on right now about Z-Max; the stuff Carroll swears by on late night TV. It actually causes engine damage.
http://autorepair.about.com/library/.../aa042801a.htm