The Disney Dining Plan is constantly changing, but when you look at the core of the offering...it can definitely be a great way to experience as much of the cuisine at Walt Disney World as you would like.
18-20% is the usual tip that we budget throughout our stay on property. Sometimes its less and sometimes it's more depending on the situations, and we can all tell our experiences on both ends of that spectrum. For this topic, it's more of a generalization and common courtesy vs individual times...because we all have those stories that we can put on TripAdvisor;)