Yes, slate is an expensive material to purchase, time-consuming and expensive to install, but a slate roof will last for a century and more with proper maintenance. Considering the long-term value of slate, it is probably the least expensive choice for your roof. An asphalt roof will need to be replaced every ten to twenty years, but your slate roof will only require minimal maintenance every twenty years or so.
The roof on our two-story Millerton house project is made of 22 gauge Galvalume, Fabral manufactured, with a high percentage of recycled post-consumer metals, making this material an ecologically sustainable choice. The paint warranty is about 45 years, and with proper maintenance this roof could last a hundred years.