There are some buffet servers who are incredibly good and warrant the full 18-20%. They are at the table all the time, checking on your beverages, clearing away used plates, etc. They deserve the full tip. But, there can also be those servers who bring you a soda and then you can't find them again.
We have always had great servers at Crystal Palace. Not so this last time (two weeks ago). Our server got to our table about 5 mins after we were seated, asked what we would like to drink, then off he went. He came back about 5 mins later with our sodas. But, about 20 mins later, when we would really have enjoyed refills, he was nowhere to be found. After waiting another 5 or so minutes, my dd went off in search of another server, who promptly brough out refills. Another 15 mins go by, we have finished our meals, are sitting there, patiently waiting for our bill. Nope, server seems to have 'left the building'!! We finally, again, asked another server if he could find our server in order to get our bill paid. It took our server about 15 mins to come out and hand over the bill. All in all...it just was way too slow. That would have been a case of our not wanting to pay an 18% tip, but less. When we were leaving we were asked how everything was. Our response? We said that the food was good, the characters were terrific but our server has been on break evidently.
Nope, some buffet servers really do deserve the full tip.