i'm a very good tipper. i don't ever really sit and do the percentage math, but i usually tip a minimum of 20-25%
if the service is "regular" or even sub satisfactory, i still tip decently. i know a lot of factors can be the reasons why service is slow without being my waiter's fault, like how busy it is, short staffed, etc.
if the service is really great, i definitely make sure that they know it.
a few weeks ago we had a dinner meeting at a restaurant. there was hardly anyone in there, but our waitress was amazing. she was prompt, polite and made sure everyone's drinks were never below half, all orders were given to the correct people without even having to ask to make sure. my order was only about $8 but i tipped her $10 because she was great.
today after our tournament some of us went to a new place to eat. our waitress was amazing. very friendly, personable, knew the menu back and front, she gave us recommendations when we asked because we'd never been there before. she had all of our drinks constantly full, cleared empty plates and wasn't overbearing at all. i coach teenagers, so it was the 3 coaches and about 7 teens and usually people dread that cuz a lot of young kids don't tip or tip well. she broke down all of our tickets for us and everything without any problems.
I paid for 3 of us and the order came to $40 and change. I tipped her $15 and wrote on the receipt that she was a great server. one of the girls asked me why i tip so much when we go out and i told her that tipping is expected, but when you have good servers who go above and beyond the minimum expected to do the job, you reward them for that.