In reply to Stairclimber:
We've rented cars on Kos thrice, to take them over to Kalymnos, as generally much cheaper there due to the wider choice. Twice it worked fine , although amusingly, once we were told that the car was insured on Kos and Kalymnos but NOT on the ferry - we reckoned that if the ferry sank, we'd have bigger problems, and if it got bumped on the ferry we'd say it happened on Kalymnos.
The third time was pretty disastrous - when we turned up at the airport, and said our holiday address was on Kalymnos, the rental company announced that the car wouldn't be insured there as they hadn't been told in advance (although we had stressed when booking through Holiday Autos that we were taking it over) and we ended up having to pay an extra €100 (which we did eventually manage to reclaim from HA). We were also told that there would be no-one on Kalymnos we could contact in the event of problems, and that if we had any issues with the car, it would be up to us to get it back to Kos. The car was crap, with brake and headlight defects, and really worn out, but we put up with it for 2 weeks.
The last time we went, we did what loads of our friends already did, paid a little extra, and had an excellent experience with "Mr Suzuki" on Kalymnos! He met us at the ferry, great little car, no issues at all, put Alison on the insurance at no extra charge, and when we returned the car to his office at the end of the trip he told us just to use it to take our stuff to the ferry and just leave it there. Highly recommended (and, no, I'm not on commission!) -
http://www.kipreosrentals.gr/
I wouldn't try renting on Kos again, although backing your rental car onto the ferry, and then watching all the Greek drivers hurtling their dented vans in next to it is ALMOST worth it for the entertainment value.
Cheers, Andy