Really BAD service gets little to nothing. I will often leave a penny on really bad service just to drive home the point that I thought about it and established worth accordingly. If the service was adequate, 15%. If it was awesome, 20%. Usually it falls somewhere in-between as I like to round the bottom line to whole dollars when practical.
For those who don't know the etiquette, you should tip on the pre-tax value of the meal, not the bottom line. Also, check the bill to see if the gratuity was automatically added in. That's often the case with large parties. My family all went out for Thanksgiving this year and the gratuity was added in. We almost missed that. Adding another 20% onto 20% would've been, what, a 45% tip in the end?