Does anyone know how much an ambulatory electrocardiogram tells you?

Depends. Unfortunately, ambulatory resting ecgs may not be adequate enough even in presence of a disease such as angina, as they only catch the moment it's taken in, which may not reveal the disease or its severity. Same for arrhythmias (irregular heart beat). Some people have irregular heart beats that come and go and it's a matter of luck to catch them with ambulatory ecg.