From the Mira Vista brochure:

It's a cut above the rest.
Only MiraVista Slate combines the elegant look of stone with lightweight durability and low-maintenance construction.

Stands up to the elements.
Advanced composite materials provide solid protection against changing weather, freeze/thaw cycles, high winds and fire.

Virtually maintenance free.
MiraVista Slate requires virtually no upkeep to stay looking its best.

All-natural colors and textures.
MiraVista Slate is made using fine particles of slate and clay to capture the rugged look and feel of natural slate.

Won’t absorb moisture.
Unlike cement–based slate substitutes, MiraVista Slate won’t weaken or soften from the effects of freeze/thaw cycles because the shingles won’t absorb water or moisture.

Lightweight alternative.
No special roof reinforcements are necessary because the shingles’ composite construction makes them lightweight.

No whitening.
MiraVista Slate won’t whiten or get chalky as it ages.