How does it work? You tell your iPhone how much you weigh, how many drinks you've had and how much time you've been drinking. If that's too complicated, you've had enough!
It does the math to estimate your blood alcohol level, and tells you whether you should call a cab.
I notice on my iPhone that there are other apps that do similar things. One called "R U Drunk" costs $2.99 and claims to administer sobriety tests.
What works for you?