Let's make a distinction here. Guiness stout as distributed in the United States tastes like ass. From a tap in Ireland, it's really good.
That's only because the atmosphere makes you *think* it tastes better. Although I'd like to go back and have a pint just to test the theory...
Since this got bumped anyway....can confirm US draft Guinness and draft Guinness at St. James Gate are indistinguishable.
I heavily tested this using a very scientific approach last year on our trip to Ireland.