I've now used Sixt across a few countries and I find they always provide a great service, with great cars and for a good price. We hired a car for 5 days and our experience was no different. Friendly staff who gave me a free upgrade into an automatic (which I'd recommend for Kefalonian roads btw) and really quick service, both checking the car out and checking it back in. Car was great and allowed us to see the whole island!
Most importantly there was no attempt to charge me for previous damage already on the car, which has happened to me in the past!