IIRC, Surface is one of the key reasons Microsoft did so well in the last quarter.
I wouldn't say it's a key profit area for them yet, and we also can't call it a success yet (as they haven't reported the total retail purchases).
[1] http://blogs.wsj.com/digits/2014/10/24/a-win-for-microsofts-...
So, Surface won't be replacing Windows/Office any time soon, but nearly a billion in revenue for what a lot of people try to frame as a failed product is not too shabby.