I know, most people put it on the roof, but you can't see it on the roof, so we've put it on the walls. I was buying gutters from a local roofing company, and their yard was filled with pallets of slate that they had removed from the roof of a bank. This was several years ago, and our plans were not fully developed, but it planted a seed in my mind. Luckily, the slates were still there when we began construction (not many new slate roofs going up), so I bought 1800 square feet for roughly the cost of vinyl siding. The tiles are about 12" x 20", and are hung with an 8" exposure. This means that 12" is covered by the tiles above and the wall is essentially three tiles thick.
We're using raw copper as a trim detail at corners, around doors, etc. It's bright and shiny now, but within a few months, it will dull to a subtle bronze, which I think will work well with the slate.