Very fast, professional, and friendly!
Sixt will be Sixt: always serious and helpful. Don't look elsewhere because many other "cheaper" alternatives will end up being more expensive, and giving you headaches. I know it from experience.
Until today, I've only had bookings in Mitte, so this was my first time at the Lichtenberg office. Great customer service, clear explanations with a big smile. My German is still limited, and I had zero issues. The person in charge gave his best so I could understand everything. I will come back for sure.