I just moved here from Colorado, and have yet to have a margarita as good as the on-the-rocks ones served at the Rio Grande Restaurant in Boulder. My first margarita in Austin was at Chuy's, and it was one of the weakest drinks I've ever paid $7 for. I thought it must have been a moving-to-sea-level thing, as I am used to consuming them at a mile high, but then I read further down the board that their drinks aren't well-respected. The best I've had so far was a Mexican Martini at Mesa Rosa. Ask for an extra shot of tequila.