I bought the DDC55 mainly to clean the glass roof. It works, but it's a bit disorientated upside down, i.e. it doesn't clean in a controlled manner like it does on a vertical surface and it never reports that it's finished.
But if it cleans for long enough, it gets everywhere. For me it's ok, much easier than cleaning by hand.

And on the outside of the glass roof, does it work there too?

I sent it back a long time ago. At first I was totally thrilled, but the window robot leaves a lot of streaks and it looks almost worse after cleaning, so I wouldn't recommend it at all.