What is your definition of good here? Are you talking about a single sheet of metal, or something over it? Because a single sheet sounds like it would be terrible insulation and really loud when it rains.
A single sheet of metal painted white should insulate pretty good in the summer, where your goal is to prevent conduction of heat from the ambient air outside and radiative heating, but not in the winter, where the heat you need would rise and conduct out the roof. So it really depends on your climate, if you live in a warm place it's not so bad, a cold place and you're going to hate it.
If you insulate it though it's not a problem, and they're more durable.