I have experience with Hertz and Europcar in Sardinia (and other events/places). Good experiences with both, in fact. With Hertz we never had issues, although sometimes the car was dirty and there was always something not right (scratches, something in the windscreen, damage to the bottom part of bumper etc). With Europcar we once returned our rental with several scratches, a hole in the rear bumper and a damaged windscreen, but never heard anything from it. I think you need a bit of luck. Also a brand new car will be checked more than one that already has some issues. We always take the extra insurance (SCDW it's called, I think). It costs maybe 100 to 125 euros, but if you divide it by 4 it's nothing compared to the problems (and possibly euros) it can save you. Anyway, still have a good look at the contract of the rental and the insurance. In most cases the interior and bottom of the car are not insured, in some cases also tyres and windscreens aren't taken in the insurance. There are cheaper insurances you can take when renting a car, sometimes from a different company (like Allianz). The rental company let's you pay and then you have to reclaim to the insurance company you chose. But I heard some bad things about it and don't trust it...
