i've used STP, gumout, and sea foam all as fuel additives. i would avoid gumout at all costs, when i ran a tank of it, my car seemed sluggish and unresponsive, especially in the cold.
if you do use bottled additives, pour the cleaner in the tank BEFORE you fill it up. that way when you fill up the tank it mixes better and it washes traces of the additive down the filler tube.