We had a really great experience with Sixt. I had read quite a few negative reviews (and many negative reviews about hiring a car in Croatia with any provider) so I was quite nervous about it all but it went smoothly no issues. We hired an automatic car with Sixt quite last minute so it was really expensive but we knew that going into things. I also went over all the T&Cs on their website so I was aware that they take a €1200 deposit in addition to the hire charge (some people seem to have hired without realising this). We booked the Sixt insurance option that gave us an excess of €825 and then we took out personal travel insurance from a provider in the UK that covered any car hire excess. We didn't take our tyre and windscreen insurance but that would also have been covered under our personal travel insurance. We felt that the Sixt prices for a zero excess and the tyre/windscreen was just too much more expense for us.
The check in process was super easy. We were given an itemised bill at the airport and Sixt added on roughly €65 for refueling the car on drop-off. I was told that there wasn't a gas station near to the airport but if we did bring it back full then they would remove the charge. I also made sure to tell them that we were going to be taking a ferry during our trip, so they added on the extra charge for that (it's a small fee applied for the whole rental per day but I had read about it in their T&Cs so was aware of it). When we got the car we were realllllllllly pedantic with checking it over and noting any marks/scratches/dents and making sure Sixt recorded them which they did. We also took photos of all the marks ourselves and took a photo of the inside mileage/fuel before we took off.
The car was excellent. It was a newer car, super clean, barely any mileage on it and drove great. We Googled and found a gas station 7km north of the airport so on our way back we refuelled the car as much as possible and then just cruised in eco mode down to the airport. The fuel registered a fuel tank :)
When we brought the car back to the airport we just drove to the same spot we picked up and the staff got us to park it and checked it over. We made sure to tell them the tank was full and to remove the refuelling charge they had added. Then we went into the office and got our final bill within a couple of minutes and it was all done.
Checked my credit card when we got home and the deposit has been removed and all is good. So we were a bit paranoid but all went well and we were really pleased with the service! I think going into any car hire with caution and properly reading the T&Cs will prevent a lot of issues for people.