It might be a fair statement that the merits of a Tex-Mex restaurant can be roughly gauged by the merits of its margarita. For better and for worse, this is certainly true of Moderno Tacos + Tex Mex, a bright, endearingly casual new spot on the western banks of Beltway 8. Bolstered by freshly squeezed lime juice -- the press is proudly displayed behind the bar -- the margaritas here could easily make a short list of the city's best, if they didn't run so perilously close to over-sweet. For a few dollars more, you can opt for a better class of tequila to be shaken into yours; it's nice to see quality offerings such as Tapatio sitting alongside the usual suspects. You might even opt for a nicely woody reposado from Tequila Ocho for a duskier spin on the classic. Perhaps you can persuade the bar to strain yours over fresh ice -- the better to control melting and dilution -- rather than unceremoniously dump the contents of the shaker into your salt-rimmed glass. It's one thing to laud the charming casualness of the gesture, but another entirely to let the drink suffer, even if ever so slightly.