Slate might accurately be termed the "king of roofing systems." It is durable, resisting hail better than almost anything else. It is long-lasting, with slate roofs in some areas of the world remaining serviceable for literally hundreds of years. It is also beautiful; regardless of one's taste in roofing, few would dispute that a good slate roof simply looks fantastic.
Real slate, which is quarried out of the ground just like any other mineral, has been used as a roofing material for centuries. The main drawback to real slate is its price tag: a roof made out of it will often cost as much to build as the rest of the house. Also, the life of real slate can be shortened by volatile climate conditions. A freeze/thaw cycle like that of the Ozarks can wreck a slate roof in a fraction of its normal lifespan.