We had a completely fine experience with Sixt at the Ljubljana airport. The car was good, came to about 340 euros for 3 days with zero excess insurance, navigation and second driver. The deposit was 500 euros extra but this was returned within 24 hours.
There was a scratch on the car before hand, we videoed and showed the man at the desk who helped us and he said this was purely cosmetic. There was a friendly member of staff at the end who did a full walk around inspection of the car and confirmed all in good order and then gave us a document to sign to confirm this. This was very reassuring.
Reading other reviews always worries me, but I think it should convince you take out fully comped insurance. I'm sure most people who have done this have had no problems.