On a renovation we recently completed, the metal(tin) roof had a stamp on the back, the installer gave a 25 year guarantee. The roof was a little worse for wear, but was not leaking. The guarantee expired in 1905.
For most of us, the expected life of a metal roof is beyond our expiration date. That is not true of most modern cedar shingle roofs which I have seen fail in less than 10 years(due to acid rain and fast growth cedar). Since the cost of a cedar shingle roof is comparable to a metal roof, I am surprised metal is not in more demand.
Of course there are many different metal roofs and their prices vary greatly. Probably the most expensive is lead coated copper and the least is either galvanized or painted steel. Painted steel comes in a great variety of colors.
One question we are often asked is: Is a metal roof too loud when it rains? There is no question that a metal roof makes more noise during a rain storm than fiberglass architectural shingles. Since most of the homes we design have spray foam insulation, sound is not a problem. If you use fiberglass batt insulation, and your bedroom is open to the underside of the rafters, it could be loud, however some people like to fall asleep to the sound of rain on the roof.
We often use metal roofs as accents. They are great for porch roofs, dormers, and cupolas to break up the great expanse of another material.