As with everything else, there are wide ranges in price, appearance, and longevity. "Unfinished" or natural finish materials such as copper, terne metal, stainless steel, "Galvalume", even plain old galvanized (G90, 1.5 ozs. zinc/sq.ft., best) require no cosmetic maintenance, and will in most environments probably outlast the functional life of a residential building. Of the "coated" or "painted" finishes, Kynar 500 is vastly superior to any other, and priced to match, but probably worth the steep initial cost, since it too should last well beyond the functional life of a residence. There are also great varieties in "style" from batten and standing seam and even shiplap style metal shingles, etc., to the more common and cheaper roll-formed corrugated profiles used for roofing barns, sheds, cabins, etc. If you go with a "painted" finish, think seriously about buying aluminum vs. steel --- it's greater longevity is likely worth the extra cost.
As has been mentioned, this is work that ought to be done by a specialty contractor with a long resume establishing experience and satisfied customers, most of whom will likely be commercial rather than residential consumers --- look for fancy architectural sheet metal work installed in your vicinity, and find out who did it, as a source of leads.
Commercial general contractors will generally be happy to refer you to favored subcontractors.
A metal roof of high initial quality should be a once-in-several-lifetimes investment, with little maintenance cost.
