The primary car rental coverage is the main reason I renew my Diner's Club card every year. They were fantastic when someone tried to steal a car I had rented while mine was in the shop. Even more so because I was using a free weekend rental certificate for the car, but had also provided my DC when I rented the car. They handled everything once I notified them -- even Fedexed me a copy of the settlement check paid to National within 24 hours of receiving the final total for the damage (broken window and cracked steering column) from National.

For those that recommend using your own auto insurance coverage, you really need to check your coverage closely. Many personal policies no longer cover rentals made for business purposes, so if you're renting the car on a business trip, your claim would/could be denied.