All in all it was ok. Car was good and we had no issues with picking up or dropping off the vehicle. The only thing I have to critizise is the nonsense what the blonde girl from the service is telling to customers. We were been told that the road between the airport and Reykjavik is the only road which is sealed, all the others are just gravel roads and that's why we should pick a 4x4.
After a trip around the island, I can say that's so not true cause 90% of the roads were sealed and in pretty good conditions.