Fuel Additives

Just a question about the octane additives you can buy to pop in your fuel really,

have chat with a skyline owning friend over the w/e, basically he was saying that about twice a month he sticks additives into the engine, said this gives him much better fuel economy in some cases another 50-60 miles. Also mention of it being better for the engine, cleaner.. etc.

So, question really, how much of this is actually true, other than the obvious cost what are the negatives... what brand additive is best to use, and how often?