I put Ondura on our home about 18 years ago myself. I have a T style roof which makes for 2 valleys. Valleys have been a real problem, leak caused damage, had to replace some roof sheating. Replaced ridge cap, the sun just ate it. The house faces South, the South side Ondura has harden and become brittle in addition to deterating. The Ondura instructions indicate that it should be painted and I belive that would prolong the roofing, I am just a little late getting that done. There doesn't seem to be a good method to keep rain water from getting under the Ondura in a Valley and while Water Dam does stop any leaks, water that pools always seems to find a place to get through. Water Dam closes up around the nails, however, sooner or later the water will find a weak nail hole and in comes water.
Metal roof will require painting at some point but will not be brittle or curled up requiring replacement like Ondura. I will not use Ondura again and will probably have this roof replaced with metal before much longer. I would suggest not using Ondura even though I think I have lasted long enough to be able to say that I have gotten my money's worth out of it compared to asphalt shingles, but then that was not the intent from the beginning.
My parents home is over 125 years old with a metal latch type roof. It's been painted over the years, I would be afraid to guess the thickness of the paint but the roof still does it's job. It was painted by hand for years, then later with people that came around the country wanting to paint barns, houses, etc. They used very long extensions on paint sprayers while standing on the ground. Job would be done in short order and good for several more years.
Since landfills don't want roofing, I don't intend to fight it any longer, I will use metal.