It gets you properly wrecked. An alternative is to start with a pint of stout, drink half, add half a pint of bitter and repeat the process 'till you're only drinking bitter. Then, if you're still standing, you go back the other way till it's only the stout in your glass. If you still know which way is up, you're doing brilliantly.
@daynewright I'm curious as to how 'black and tan' came to be associated with St. Patrick's day in America, given the term is a nickname for the special reserve constables of the Royal Ulster Constabulary (also known as the B Specials) - that is part-time policemen for the British when they still ruled Ireland. They were mainly known for being violent and profoundly anti-Catholic, so I think something got lost in translation when the term crossed the Atlantic.
Finally got the joke, black and tans knock you out!