We are all conditioned to think of metal and water as a combination that leads to rust. If the metal is unprotected, it will deteriorate; it will rust. So when someone suggests roofing with metal, a major concern is how it will withstand the elements, especially rain.
Here are two things to consider if that is a concern of yours. First is durability. Metal roofing today is coated with an aluminum-zinc alloy combination that protects the metal sheet edges and helps it resists corrosion. Metal roofing treated with the aluminum-zinc alloy dosen't crack or peel when exposed to rain. The protected sheet is them painted with a protective paint, often Kynar. Kynar paint protects and preserves the steel base; increases the roofs reflectivity; and helps preserve the original color for decade.
A second consideration is your roof's pitch. Most homes have a roof slope of at least 3:12. This means that for each 12 inches of run (eave to ridge) the roof rises up at least 3 inches. With a 3:12 slope, rain runs off the roof and doesn't gather or pool.
Ask your contractor if the panels he plans to use on your roof have been treated with the aluminum-zinc alloy coating, and if the coated sheets are painted with Kynar.