When you first launch the application, you'll enter some personal information, like age, weight, and gender, so the application can calculate your BAC correctly. The main display shows four drinks on a table. You can change the type of beverage by double-tapping and selecting a different one -- the icon will also change. Each time you drink a beverage, you drag the icon off the table and onto the screen. The BAC, Carbs, Calories, and number of drinks will be updated in the top-left corner of the screen. As you add drinks, these numbers will increase. The red line across the screen indicates the legal limit -- when you're above this line, a new button will appear called "call a taxi." When tapped, this button will launch the Maps app and search for taxis.
I love subtle UI features, and this application definitely has some: from the icon sets for beverages, to the sloshing liquid when you tilt the iPhone. The application is really great, and I would love to see the developer create a caffeine calculator that does the same thing. I would like to see the ability to input a number for a designated driver, that way you don't have to fumble through your contacts or call a taxi. Got Your BAC is available from the App Store for $2.99 (US). And, remember, the best thing to do is to not drink and drive.