If you decline the car rental company's insurance and charge the rental fee to your Amex, then Amex has a free car rental insurance plan. It's essentially secondary to any other coverage you carry, so if you wreck a rental car and already carry collision insurance, then it hits your coverage first and the Amex plan just pays your deductible.

They've also got an opt-in plan where any time you rent a car with your card they charge you a $25 flat free, and then that buys you coverage that runs primary so you don't have to claim against your own car insurance at all.

Yes, this is what I meant by super cover. Essentially you just rent without any additional coverage and Amex will cover anything not covered by the rental agency's insurance. This saves a good $20/day at least, meaning if you rent cars for just a few weeks per year the fee will be worth it, when considering the discount as well. I've racked up about 7-8 weeks of car rental so far since New Years, and thanks to Amex I saved a bunch of money on it, probably more than the fee.

Another good feature is I've sometimes had to return the cars in different cities, and every time they waived the fee because of the preferred status as a card carrying member. These things add up for sure.

As far as I know, the insurance also works with any agency, so not just Avis and Hertz, so there's money to save even if you go for a more "fun" agency like Sixt, with nicer cars.

Disclaimer: You'll want to verify all this with Amex though, but this is what I've been told and what I've read in the ToS.