I made the mistake to use Uber X in Düsseldorf. Like in other places on this globe, they quoted me a (good) flat fare in the App, EUR 5,55, which was close to half of the taxi price I know pretty well (EUR 9,00). In the end I received however an Uber invoice per Email, charging me a base fare of 2 EUR and extras for time an milegage. From what I would guess exactly the fare based to the citycouncil's directive for "Mietwagen" and close to double the EUR 5,55. For that result I had to wait close to 10 minutes and sat in an old Toyota. I did not get a VAT invoice, to could not reclaim the 7% included in taxi fare. At the end the old Toyota was more expensive than a regular taxi. No need to do that again but a need to make that public to warn others.