depends on the additive. if you're talking octane boosters, yeah, 99% of them are. The only ones I've seen that work well are the Torco concentrated race fuel line; the only reason I know they work is basically people using them sucessfully with their forced induction cars and race gas tunes - but only on pump gas and the torco additive. Again, they wouldn't do anything on a car not tuned or not needing that kinda gas tho.

I was stuck in BFE once with my mustang that needs 91, and only 87 was available (new mexico...)... the people in the gas station looked at me funny when i bought all of the octane booster they had on the shelf... my car still pinged on the way home if I got on it this was after about 9 bottles dumped with about half a tank of gas... so yeah, they don't do anything IMO...

Additives that clean stuff do help, though not as much as professional cleaning - mainly injectors - it's generally much better to take it in for a professional cleaning job where they hook up the injectors and run concentrated cleaner through them, rather than putting the stuff in the tank every fill up.