It depends on the service. Average service gets 15%. Good service is 20%. There have been a few times where the service has been just superb and I've left a fairly substantial tip (almost half the bill).
As a former waitress, I know how hard it is to get everything firing on all cylinders...you are at the mercy of the kitchen most every night, the service staff (bus and hostesses) and the customer at all times so good service can hard to provide. When I get it, I know what it takes to get it and I reward the waitress/waiter appropriately. Also, I know that the more service help there is (bus boys/girls, hostess, bartender, etc.), the more your tips get diluted. When I waitressed, the standard was 5% of your total to the bus boys, 5% to the hostess, 10% to the bartender, and 10% to the sommelier. So off the top I lost 30% of my tips to others in the restaurant. Another reason that I'm a bit more generous than the majority.