Do a search we've been through this a few times already.
The consensus was if its too good to be true it usually is! The crazy info-mercial with the car driving around in the California sun with no oil just a bottle of prolong is a joke. Come on! I dare you to do that to your car. Also a lot of these additives have not been through a lot of long term testing which means that the effect on an engine over time when they break down is unknown.
Finally, most of these companies are under investigation by local consumer affairs commissions, I suggest you call your local one up.
just my 2 cents. And get ready for some others.