I've only rented with Sixt once at Budapest Airport. I've had no problems with them. The Opel Astra came quite clean and with a low milage. They did upsell me a category upgrade but in retrospect that was a good choice. The service was really not outstanding but they did not make a fuss.
It's Eastern Europe , don't expect wide smiles here. Best you can get is efficient and professional service. And that's exactly what I had here.
On return it took a few minutes to find the man in the car park and he wasn't the friendliest either but I haven't been charged for any bogus scratches.
They did hold on to my deposit for quite a while on my credit card so that was a bit of a nuisance but considering the rate I got from Sixt it did not bother me that much.