Gibsons are tasty. The cocktail onions give a bit of a different character to the drink than a twist or an olive. One story about the name has to do with a party promoter by the name of Gibson who had the waiters bring him a drink with a cocktail onion to distinguish it from all the olive-garnished drinks on the tray. The trick was that his drink was really water, allowing him to stay perfectly sober and in control of things while all his guests went two or three sheets to the wind. Another story has to do with a magazine illustrator named Charles Gibson who asked a bartender to make him "something different" back in the 30's. I checked the Savoy Cocktail Book and the Old Waldorf Astoria Bar Book, and both of those give Gibson recipes as a martini (equal parts gin & vermouth, as was the style at the time, i.e. pre-Prohibition) with a twist.

Honestly, I very very rarely order a Gibson at a bar. usually, my order is a martini with a twist (as it was earlier tonight). If I do want a Gibson, it depends on the bar. I'll order it by name in a place where I know the bartender is into classic cocktails, while otherwise I'll simply order a "martini with cocktail onions".