hello,
the best place - in my opinion:
Unternehmen Mitte, Gerbergasse 30
you have to go inside between "fumare non fumare" - and there is this, big space, you can sit there as long as you wish (no one ask you to leave when you finish your latte macchiato, which is very very good anyway).
my favourite place for working.
hope you will like it
Agu