Ladies and gentlemen, whenever you go out to a restaurant, do you leave a tip? How much?

20-25% always. If the kitchen is poorly run or the food not up to standard, it's not your servers fault. She/he are responsible for getting your orders correct and to you as quickly as possible. They're there to be attentive but not overwhelm.

I might, on very very rare occasions go lower than 20% if the server is inept or has a bad attitude.