Coming from a Volvo where it is standard I saw it as a must on my F11. I use it every day when it is below 5 degrees celsius. When activated from remote, which goes over radio btw, it will run for 30 minutes. When its arround -5 C its enough to activate it 10 minutes before leaving, voila, all windows are defrosted and car is warm. I am working as a IT consultant, so every minute that i can spend at customers is ££. Therefore I earned the cost quite quickly not having to spend 10 minutes every morning removing ice.
Yesterday we had -18 C, and after 30 minutes completely warm car.