We run 2 courtesy cars, which we insure for customers use.
We found it easier and less hassle than having to check insurance details etc.
Our main problem comes with fuel.
Nearly all customers refill the amount of fuel they have used, but some think its fine to return the car almost empty and not have to pay for any fuel they have used.
Do keep a log of who has used the vehicles and on which dates etc, and do daily checks for damage etc.
Hope that helps
Andy