Party Girl Party Boy

Party Boy or Party Girl (depending on the player character's gender) is a perk added to Fallout 3 with the Broken Steel add-on and Fallout 4.

Fallout 3
The player will no longer suffer the withdrawal effects from an alcohol addiction.

Fallout 4
With the doubling effect from rank 2, several alcoholic drinks become competitive with chems in their effect, except alcohol is ostensibly cheaper to come by and addiction immunity comes built-in to this perk.
 * vodka and bourbon become potent health boosters; potentially very useful on survival where the instant effect of bonus health or bonus endurance is preferable to the slow regeneration of healing.
 * wine provides a significant AP bonus
 * beer and various Gwinnett drinks can be more effectively used for bartering benefits and speech checks
 * And the melee damage boost of all alcoholic drinks becomes more significant.

The rank 3 effect does not show up as part of the item's PIP-Boy description, but it can be verified by either looking at your SPECIAL stats after consuming some alcohol or looking at your active effects.