My boyfriend and i used Sunbird for a 3 day hire, we both thought it was quite reasonable for us.
Im glad we hired the car as we saw so many places we had hoped to go and see. Our car was a Chevrolet Matiz, it was unbelivable really, only being a small car it got to places without no fuss. (Wouldn't recommend going up to Mt. Aenos in it though)
All in all the company is professional and would recommend.
Although we hired a jeep from here (only place the age was 23 and my boyfriend is 24) the excess is huge, a whopping €350 and I can assure you they go over the car with a fine tooth comb, luckily so did I went we hired it, and there was no damage on the return! We also hired a moped and again the excess is a whopping €205 and we later found out you are not insured on a moped.. but I know you should never hire a moped on holiday!! I would recommend Greekstones and just a small car for the duration of your stay if you are not in Skala (we stayed on the outskirts) and we would pick it up from the airport. Jeep was nice but not really needed in Kefalonia!