I tip 20% for good service. In an expensive restaurant, I'd go to 25% if both the service and food were exceptional. On that $100 tab, I'd tip $20.

I don't "stiff" or under-tip waiters. I did that job myself when I was a kid. I know how hard they work and know they're factoring in folks who routinely don't tip enough. If something goes bad wrong at a restaurant, we'll diplomatically complain to the floor manager, and down here that inevitably results in an adjusted or complementary bill. I still leave a tip on what the total would have been if the problem weren't with the wait staff (usually it's a problem in the kitchen). More and more, all the restaurants we regularly go to have implemented surveys or some sort of customer-service feedback mechanism, which seems to have really helped assure good quality service and food. We live in a nice area that's just flooded with restaurants, so they're all competing for all they're worth.