Good tequila has a subtle, melon-fruit aftertaste to it, and a mild sweetness. Bad tequila just tastes like artificial sweeteners mixed with fire.
Generally, tequila has a unique earthy flavor with an alcohol kick. Dependent on where the agave was grown and the style, each tequila is slightly different. Blanco tequilas offer the purest tequila taste, with an earthy, semi-sweet, distinctly agave flavor.
Aged tequila is the smoothest, with very little alcohol taste. Although it still tastes like agave, its flavor is dominated by smoked oak, vanilla, caramel, and spices.
Many tequila connoisseurs will swear that the best way to drink tequila is to sip it on its own. If you do this, you can order your tequila chilled, on the rocks (ice) or straight up, but most prefer it chilled. Do not order it with salt and lime, that is only for shots.