The Fallout manual erroneously states that it increases Rad Resistance by only 10%.
Fallout 3 and Fallout: New Vegas
With this perk, one's radiation resistance improves by 25%.
The necessity of this perk is questionable, since it is trivially easy to temporarily boost radiation resistance to the maximum with Rad-X and various radiation-resistant armor. Combined with the fact that there are few areas with a high enough radiation level to warrant such resistance, there is no pressing need to take it. In Fallout 3, the perks gained from The Pitt will grant 13% radiation resistance in total, the Cyborg perk will add 10% more as well as providing other useful effects and at the same time, and each point in Endurance nets a natural resistance of 2%. All power armors (not including tribal power armor and Ashur's power armor) increase radiation resistance as well (ranging from 10 to 25%). Even if the player should encounter heavy doses of radiation, Rad-X and RadAway is of no shortage, and an advanced radiation suit can also be obtained. Though there is less Rad-X and RadAway in Fallout: New Vegas, there are fewer areas of dense radiation, in turn.