According to testing, they're apparently an octane booster.
http://tsikot.yehey.com/forums/showt...397#post628397

There's no solid evidence that they leave any "coating" on the combustion chamber, and, as I recall, most of us have a name for that coating... carbon deposits...

Still, an octane boost is a good thing, but whether the cost is justifiable or if this is any better than switching from regular unleaded to premium is a judgement call. I still wouldn't trust anyone that claims deposits on my combustion chambers are a good thing.