I've just been on my first trip to the states and I was most impressed at the quality of beer they have over there. Admittedly this was due to my exceedingly low expectations -- I was expecting it all to be Coors or Bud Lite, and there was a fair amount of the stuff kicking around, but there was also a decent micro culture.
The two bars I recommend are Rogue Ales and Gordon Biersch, both in San Francisco and both tied to similarly-named breweries. Rogue had far more choice than you could get in a real ale pub thanks to gas delivery - over 30 beers on tap. Beer of the trip was Gordon Biersch Maerzen - a slightly fruity, reddish coloured, lagerish beer.
Worst beer of the trip - ice-cold sparkling London Pride! :(