Since we are at it, if it has a worm at the bottom it is not tequila. The little worm (gusano de maguey, a creature happily living inside the fleshy leaves of the agave) is found in mezcal.
Notice that Tequila José Cuervo sucks royally. Good brands are Herradura, Centenario, Cazadores y Don Julio.
Of course if you only have José Cuervo on hand, you might as well use it to make yourself a margarita, since what is bad cannot be spoiled.Forget the salt-and-lemon on back of hand routine: Mexicans don't do that. Good tequila should be drunk in a small cylinder shaped glass, at room temperature, and in small sips. Treat it like a good cognac, at tequila will be nice to you.
More amusing details: the glass is called a caballito, meaning "little horse". Maybe in ancient Mexico horses were small and cylindrical. The practice of aging tequila in wooden casks is a rather recent one; in the past tequila was drunk almost exclusively in its blanco condition.
It is also interesting to observe that tequila in Mexico used to have rather low-class connotation; fashionable and rich people would drink imported (and expensive) liquor, like whisky or cognac or vodka. Thanks to an effective marketing campaign, tequila managed to refine its public image from drain-cleaner to fine spirit; a similar transformation happened recently to grappa. The success of tequila outside Mexico has been so impressive that the tequila distillers have reported increasing difficulties with procuring the the required blue agave (maguey azul).