You see this variation of a regular margarita at many restaurants these days. It's becoming so prevalent that you can now go into just about any bar and order a Cadillac margarita and the bartender will automatically know you talking about tequila, grand marnier, brandy, plus a sour component.
This is just an upgrade from a regular margarita but it does make it taste better...if you do it right. The key is to use good ingredients. 100% agave tequila, Grand Marnier (or Cointreau) instead of triple sec, a nice brandy (or cognac if you really want to go upscale) for an added kick. Lime or lemon juice in place of sweet and sour mix.
2 oz. - premium tequila
1.3 oz - Grand Marnier or Cointreau
2/3 oz - brandy or cognac
2 oz - lemon or lime juice (or 1 oz. of each), fresh squeezed
Mix all ingredients in a cocktail shaker over ice, shake and pour over ice into two salted-rim margarita glasses.