I have had fantastic service...and I have had really really really crappy service.
I'm not hard to wait on, I don't ask for much (I'm not one of those people who want to replace everything that they order...can I have x instead of y)
Anyway, if they are good, 20% and up (the most I have ever tipped was 150%, but that was REALLY exceptional), but look out if they are bad, I have left notes before.
I take into consideration that if the food isn't good, it's not the Waiter/Waitresses fault, but if my salad plates are scorching HOT, that's something they could control. I had a waitress go on her smoke break while waiting on us....there's were the note came in.
I had another jerk complain about the tip...(I was having a business lunch and 3 of us left over a 40% tip, well more than he deserved) we were in such a hurry to leave, when checking out...we said keep the change...because we didn't want to be late for our meeting...well he thought that was his tip and started yelling at us. My buddy goes to the table, takes the money and told him that this is what he lost....