I have a friend that will leave huge tips if he thinks the girl is good looking or if she gives him a smile and a bit of attention, he probably didn't get enough attention when he was young... ;D
I'll tip depending on the quality of service provided, though, usually about 10-15 percent. I don't believe in leaving good tips for bad or mediocre service, but will still leave a little for the kitchen staff, because they shouldn't have to short changed for bad service. However, if I am at an establishment where I am a regular, I'll tip a bit more. If it is obvious that the server is new, I'll be more lenient with them, but if it someone that knows what they are doing and continually gets things wrong, mixed up or has forgotten something, then I'll let them know and it will be how they handle the situation and treat me and/or my party afterwards that will determine their tip...