While minimally useful in Fallout and Fallout 2, Mutate! is a reasonable perk in Fallout Tactics, where it can be used to swap out unfavored traits from NPC squad members or add a trait to those who have none. Note if a character already has one trait, it must be exchanged for the new one; the empty second slot cannot be filled.
Behind the scenes
The image for this perk is a reference to Marvel Comics' Dr. Bruce Banner/The Hulk. The image of the split personality always appeared at the end of the intro scene during the television series, which ran from 1978 until 1982.