I always tip at least 15% (15 is for terrible service... though if it's HORRIBLE I will do less... that only happened once). If I see wait staff super stressed out and dealing with kitchen mistakes, etc, I usually up the tip significantly, even if my food was wrong/slow, etc. I've never worked front of house but I've worked back of house as a sous chef, and I will say that the waiters got the brunt of our mistakes. The worst is when those mistakes are actually due to management/ownership. I remember being at a Chili's once and a totally out of line customer reaming the young, young waitress-- who was in tears. For something that was NOT her fault, at all (not anyone's actually)... I had a $50 in my wallet and left it for her. I hope it cheered her up.
We did go to a restaurant in Salt Lake City... the Copper Onion... OMG. The waitress had a full section but she was still SO attentive, and knew EVERYTHING on the menu intimately... knew the best wine and liquor pairings, etc. I LOVE places that make you feel like you're the only ones there! This place was a splurge but I gave her like a 40% tip anyways, because it made our vacation-- years later, we STILL talk about the Copper Onion and our waitress, Janelle.
We used to live near a locavore restaurant that when it started, was a GREAT deal-- $120 for BYOB cork fee, two appetizers, entrees, and a dessert. It started getting a lot of buzz in Zagat and Bon Apetit, and the NYC snobs started coming in droves. Their service WAS phenomenal... you'd go once and you were a "regular" for life. Then they started jacking the prices up and seating more people than the wait staff could handle... it was really sad to see. The last day we went, that same meal cost us over $200 and our waiter, who we'd gotten to know well over the couple of years, was MISERABLE. Poor guy.