I first saw these at least 10 years ago and was wondering too. If you Google around you will see that certain brands boast a long life. But all the details must be done right by qualified installers. See, for instance, this post from a large roofing company. (Scroll down.)

Hi. I can't give you any durability/longevity answers yet, but that is exactly what we're putting on our 1838 double brick cape next spring. We'll be putting Inspire (by Tapco) slate type on. I'd be a little concerned if you have to hire someone to apply them, unless they have prior experience. My DH is makes his living doing roofing so we'll be DIY. Our wholesaler was able to give us addresses of buildings with these and Ecostar (by Carlisle) which we looked at. IMO, the Inspire slates were much more realistic. Good luck.