Bad service = no tip, complain to manager
Average service = no tip
Good service = small tip, say 10%
Excellent service = good tip, about 15-20% depending on how much change I have on me
Built-in service charge = no additional tip, even if I'd normally have tipped more than the built-in amount. I frequently tip 10-15% for average service as well, but don't feel obliged to.