Since 15-18% is considered standard, this advice would be generous, at least in my part of the US. kas23 is right that it would be acceptable in any restaurant. It most likely would be very appreciated, particularly if the service is special. (I'm not trying to start a flame war about proper tipping levels, honest.)
As a member of the food service industry (albeit one with a generous salary), I'd like to point out that for most servers, the tips are pretty much what they make. Most places pay $2 an hour plus tips, but my guess is that the $2 ends up getting withheld and the server/bartender ends up with whatever he/she makes in tips. Consider for a minute if your boss or your customers got to decide what percentage of your salary you make each day. That's basically what happens with a server.
That being said, I'm not averse to tipping based on service (i.e. 10% for lousy service up to 25% for great service, averaging out to 20% or so).