We hold lan parties around 4 to 5 times a year, usually over a weekend, and people are usually armed with atleast a case of beer (8 litres), and a cheap bottle of liquor. It's pretty damn fun.
Alcohol doesn't have a big effect on performance, until you reach a certain point. Usually we play RTS's before people are absolutely hammered. Shooting a gun is easy when you're drunk, micromanaging is not. Then again having someone absolutely hammered in RTS can create some hilarious wtf moments.
Getting stoned isn't really good for gaming. You just get overwhelmed by everything that's going on in screen, don't really function, and want to turn the monitor off and listen to the menu music. And eat whatever food you brought with you.
Alcohol = fun
Friends = great
Games = great
Alcohol*Games*Friends = fun*great^2 = good times