I have to say, that I have only had one bad experience at my local supermarket, Safeway (once, when I was 37 years old, one of the cashiers asked for my ID when I tried to buy some wine - I don't carry it with me, so I made a small fuzz about it, and they finally let me buy it. I'm sorry, I may look a couple of years younger than my age, but I have no hope in hell of looking 20).
Kathy, my sister, however, has been having the worst luck at Ralph's in Chatsworth (in the San Fernando Valley) - where they are making huge fuzzes about ID's. It seems more a matter of trying to annoy customers than of legitimately wanting to make sure the customer is not underage. Don't these stores realize that that's how they lose customers?
Anyway, here is her story