Never known anyone who doesn't leave a decent tip. At buffets, the service is just more diffuse. It's not just how well the plates are cleared, but how promptly cocktails, dinner drinks are served and how well the buffet is stocked. Generally, you aren't tipping your wait person, but the tips go into a pool.

I tend to tip on the high side for personal service at the table (20% rounded up) and on the low side (according to my colleagues) at buffets (10%).