We went with galvalume on our cabin. I designed it basically like this:

5/8" plywood T-111 decking over the rafters
Vapor barrier over decking
Double 2x4 blocking on 24" centers over that perpendicular to the rafters
R-15 insulation in between the blocking
1/2" decking over all of that
Waterproof barrier over that
Metal roofing screwed through the decking and into the blocking.

It's sort of a sandwich type construction similar to an Alaska roof. I did it this way because my rafters and the pretty side of the T-11 are exposed to the bedrooms below.

I don't know of any downside to a metal roof as of yet. We are sleeping in the cabin now in one of the upper bedrooms and I haven't noticed any more noise with that set-up when it rains than with our traditional shingle roof at home.

One thing about a metal roof is that you can collect rain water off of it for drinking, watering plants, etc... Whereas a shingle roof you aren't supposed to because of the chemicals it leaches. Until we got city water up there we had planned to collect rain water from the roof into a large cistern for our main water source. All houses in the British Virgin Islands are set up this way with the whole basement being giant under ground cisterns.

When we build the house at the beach we will go with a galvalume roof also. I don't see how a galvalume roof would be any hotter than a black asphalt roof. If anything it would be cooler. Lighter colors reflect heat and darker colors absorb it.