Fabulous service by Grzegorz! Recieved a nice upgrade as Sixt Platinum member. Recommended branch!
Because I have now read previous, negative comments...
The English skills of the branch employees are fine - no communication issues there.
Yes, SIXT (and other rental car companies) advise to get insurance for sand/gravel - so check with your credit card company if they cover this already. SIXT did explain all insurances but at no point pressure me to buy insurance that I already have.
They do have offers with kilometres restrictions and without. So be careful what you book. I have booked without restrictions, and everything was fine.
The car I received was an upgrade, in perfect condition and had about 7800km on the clock. I suggest to clean the car before you return it - A) good manners (you do flush the toilet before you checkout a hotel, don't you?) and B) you can spot scratches/dents easier that way and possibly avoid discussions. Many petrol stations do have water hoses free of charge, so this should make things easy.
And a general advice for the big rental car companies / hotel chains: book directly through their official channels