We pretty much always have a six-pack or two shared around the table among 4-5 people. We drink in moderation, most of us never having more than 3 beers. Interestingly enough, when I'm DMing, I tend toward the 3 beer limit more than when I'm playing. Game flows better after the 2nd beer, as far as I'm concerned.
There are only 2 times I can remember a game suffering because of alcohol. Once in college, one of the players was pretty skunked but this was during the 1st Gulf War and he had family in Israel as it was being targetted by scuds. So he was, shall I say, a little mental for a few sessions drunk or not. Great guy though under normal circumstances.
The other time was when I was in grad school and the reason the game suffered was because there were 6 of us trying to finish off a couple of cases of beer and there were other people in the room. So, basically, we were gaming and a sort of party broke out and we never really got much done on the gaming front. Had a nice party though.