We always tip 15% minimum, before tax. For exceptional service, which we've encountered a few times, we'd leave 25%-30%. Service is one thing, but if the food doesn't taste up to par or it gets to the table cold, then the server shouldn't have to bear the blame. I know in some establishments the tip is split among the cooks, bussers, hosts, and expediters, so the wait staff end up with a small amount at the end of the day.