I always tip at least 15%. If service is incredible I'll go as high as 25%.

That being said, if the service I recieve is not up to par with what I think it should be, then I'll deduct it from the tip. If I sit at my table for 15 minutes with an empty glass, well then that waiter isn't going to get 15%.. if my waiter is horrible, I won't even leave one (which is rare.. only happened once or twice).