Basically, the Amex car rental protections seems to provide similar coverage as the Loss Damage Waivers offered by rental car companies.
When it came to covering the cost of repair or vehicle replacement, American Express car rental insurance is definitely cheaper, especially when you rent cars a lot.
However, in doing a little research through different travel related forums, I found a common issue with regards to Amex Car Rental Protection that I think you should be aware of.
In the forums, some renters spoke of being charged with loss of use fees (a charge while the vehicle is unable to be rented) and administrative fees when using american express car rental insurance and stated that Amex would not pay for that, unless the rental company could prove they had more than 85% of their fleet rented during that time and the costs to process the accident (thus the administrative fees) were not their normal cost of doing business.
Basically, the issue seems to be that Amex car rental protection will only pay loss of use and administrative fees if the car rental companies provide a fleet utilization log; but from what I could determine by the issues that I read about, the car rental companies were refusing to provide such information. You may want to check into this issue with both Amex and your rental car company of choice. It would be helpful to know in advance of what you could expect, should damage occur to the car while you're on a rental and you are using american express car rental insurance rather than purchasing rental car insurance coverage from the car rental company itself.
When I checked with Amex, they stated that because coverage differs by Card, you should refer to the description of coverage, either in your Cardmember Agreement, or in information provided to you (in updates), for benefit details.
They also stated that coverage is a little different for the Car Rental Loss and Damage Insurance Plan vs the Premium Car Rental Protection Plan. They would not give me information on the differences; they stated that you should check your cardmember agreement.
One statement on the Amex website that I noticed was
"While on vacation, you have a fender bender and damage your Rental Car. You call Premium Car Rental Protection, and we do the rest."
It's important to know that you would still need to contact the rental car company and fill out an accident report form.
Note: Credit card companies can modify member benefits at any time; I make every attempt to maintain up-to-date info, but always check with an individual company to verify any policy or benefit coverage. See: Disclaimer
Pages from this site, relating to this topic:
Car Rental Insurance Overview
Car Rental Credit Card Coverages