With a Capital 'G'. So good that it surpasses the qualification of 'beer'.
Guinness is a type of (some say the textbook definition of) stout. Like a typical stout, it is dark in colour, almost black and has a mildly bitter mellow taste. The creamy head that is commonly associated with a pull off the tap can be approximated using patented "widget" technology in cans. When pulling a pint off of a tap, it is customary to fill the glass roughly 2/3 to 3/4 full and let it sit before pulling the head.
Contrary to popular belief, Guinness may taste heavy, but actually is very low in alcohol and has less calories per serving than Coors Light making it a perfect anytime drink. Another bit of trivia- It is law in Ireland that if a barkeep tips or spills the head of your Guinness, you are entitled to a fresh one.
"Guinness is a wonderful stout."
"Two pints of Guinness, please"
"No, I will not trade you a Guinness for one of your Heinekens"