I should probably just stay away from this thread, as I have served while in college. I know a few comments will make me want to go on a rant.
Anyways, I tip 20% usually.
Oh, and if you're food gets ****ed up, use a little brain power and realize the servers aren't responsible for everything that goes wrong with your meal. Servers don't get paychecks, and it's a little disheartening when some dip**** cook screws up, you get yelled at for it, and you'll not be receiving a tip as well.
Most of the cooks I've worked with could give a **** less about their jobs. Restaurants will hire any bum off the street to be a cook, and it usually shows.