I'm far from a teetotaler, but I have to agree with Bitt's assessment that, unless you your dinner is 3+ hours long, or unless those margaritas are exceptionally weak, you've, at least for a time, crossed 0.8, which is fast becoming the new national standard for legally intoxicated. Now, by the time you've finished the meal, hit the road, etc. I seriously doubt you're *signficantly* impaired when you drive, but at the same time, you never really know how strong they're mixing them, and it's a non-arguable fact that impairdness begins with the first drink. Having a device that proves you're at a certain BAC seems to just be a false sense of security. That's not to say you should call a cab if you have a single beer with dinner, but know that, even with a single beer, you're partially impaired (and the law recognizes this, by allowing you to have > 0 BAC.)

Edit: Incidentally, my new avatar has nothing to do with this discussion, but it is remarkably appropriate.