I never once had a "CC broken, cash?" question, granted, in germany it is also more common to pay in cash anyway.
The apps involved weren't bad either. You entered your location (or had it located via GPS), told the destination and it would give you the exact price that you could either pay directly at the taxi (before you drive) or via Paypal/CC/etc.
It was quite pleasant though apparently not very popular since a lot of people simply prefer public transport or their own car here.
