Despite paying for the premium insurance package, when our Sixt rental vehicle suddenly died, Sixt did not deliver roadside support. I called multiple times, waiting on the phone for 20-40min. each time. Every time I reached a representative, they assured me support was on its way, then hung up. This lasted for dozens of calls over 15 hours, stranded in a parking lot miles from our hotel. Finally, after countless phone calls and hours of vacation wasted, one Sixt representative had the integrity to admit that in fact, Sixt was not able to do anything to support us. She personally arranged for a private tow car to pick up the vehicle. My wife and I then had to pay for our own taxi to the nearest airport, ~1 hr away. This cost €130 of our own money. When we arrived at the airport to pick up our new vehicle, Sixt gave us a downgraded vehicle and did not refund us the premium price we paid. Bottom line: we paid a premium price, but got very poor service.
*UPDATE*: Sixt global customer service provided great recompense when I contacted them to share this problem.