Some recent experiences:
Location: STR
Status: Diamond
Reserved: Audi A6 Avant 6-cil.
Received: Audi Q8 TDI (could also take a Volvo XC90 petrol)
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Location: STR (a few weeks later)
Status: Diamond
Reserved: Audi Q5
Received: Audi Q8 TDI (could also have an S6 Avant at no extra costs)
In general the Diamond treatment at STR airport is very good. They treat me very well.
And the Diamond cars are always parked just outside the terminal so need to walk to the regular Sixt Parking.
Next week I am renting for 12 days in AGP, let's see what they give me there.