I've found a source on the internet and they seem to think that it is better than granite.
Slate is great.
I'm sure it's a matter of taste as much as anything else. I have no doubt that slate, as described on the site you provided, would be the equal to granite, or better - depending on your taste and how you intend to use the surface. Slate is a superior surface in several critical ways. Slate is fine-grained, relative to granite, and also much more uniform in composition. What this leads to is a material which is very durable, both to mechanical and chemical abuse.