CDW coverage by credit cards is very 'caveat emptor', so to speak.
I started a thread back in October to get some input on the American Express Premium Car Rental coverage:
While I was trying to get more detailed information on the Amex offering specifically, there are data points in several posts concerning other coverage options. There's also at least one post that also mentions that Visa is indeed better than Amex when it comes to coverage for loss-of-use, which has been mentioned here.
I was able to find quite a bit of information on other sites, FW in particular, since a majority of folks there seeem to be travelers. 
@TiggerDat wrote:Just to summerize; there are two types of coverage secondary and primary and almost every card has one of these.
Secondary will mean that your insurance company will pay first and the credit card will pay second.
Primary is the credit card pays for all.
Almost every Visa, MasterCard, Amex, and Discover has secondary coverage, regardless of the level of the card. Some of the card issuers may not be aware of what they offer. I asked Comenity about it once and they told me no, but when I checked the Visa website they told me that the card did have it.
You might have to check with your insurance company to know if you are covered.
I don't think the Amex, as it was explained to me, is actually primary for the $25 fee. I know it does offer some added benefits over secondary insurance, but I think primary insurance still does a better job. However, I might be wrong or the person, from Amex, who explained it to me might be wrong.
Yep, it does seem to be primary if you sign up for the Premium coverage:
https://www295.americanexpress.com/premium/car-rental-insurance-coverage/home.do
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@TiggerDat wrote:Just to summerize; there are two types of coverage secondary and primary and almost every card has one of these.
Secondary will mean that your insurance company will pay first and the credit card will pay second.
Primary is the credit card pays for all.
Almost every Visa, MasterCard, Amex, and Discover has secondary coverage, regardless of the level of the card. Some of the card issuers may not be aware of what they offer. I asked Comenity about it once and they told me no, but when I checked the Visa website they told me that the card did have it.
You might have to check with your insurance company to know if you are covered.
I don't think the Amex, as it was explained to me, is actually primary for the $25 fee. I know it does offer some added benefits over secondary insurance, but I think primary insurance still does a better job. However, I might be wrong or the person, from Amex, who explained it to me might be wrong.
@humuhumunukunukuapua'a wrote:
Just spoke with the coverage arm of Amex this week and they stated that if you don't have the coverage through your own insurance to serve as primary, their insurance steps up and serves as primary as long as you pay for the rental in full with an Amex card that has the coverage. Each Amex card is different and not all rules apply to each one.
You also don't necessarily have to pay extra (the $25 premium coverage) to get the Amex coverage to step up as primary. Amex basic coverage (free) through certain Amex cards (you will want to check with them as it is specific to certain cards) also becomes primary if there is no primary. The premium coverage is a premium add-on that covers extras including more expensive vehicles.
Also, even when a premium card (i.e., Chase, Amex, Citi, etc). has CDW coverage. it does not necessarily cover every expense or aspect of a situation. There are certain exclusions and exceptions. Important to remember. 
@Anonymous wrote:\@TiggerDat wrote:Just to summerize; there are two types of coverage secondary and primary and almost every card has one of these.
Secondary will mean that your insurance company will pay first and the credit card will pay second.
Primary is the credit card pays for all.
Almost every Visa, MasterCard, Amex, and Discover has secondary coverage, regardless of the level of the card. Some of the card issuers may not be aware of what they offer. I asked Comenity about it once and they told me no, but when I checked the Visa website they told me that the card did have it.
You might have to check with your insurance company to know if you are covered.
I don't think the Amex, as it was explained to me, is actually primary for the $25 fee. I know it does offer some added benefits over secondary insurance, but I think primary insurance still does a better job. However, I might be wrong or the person, from Amex, who explained it to me might be wrong.
@humuhumunukunukuapua'a wrote:
Just spoke with the coverage arm of Amex this week and they stated that if you don't have the coverage through your own insurance to serve as primary, their insurance steps up and serves as primary as long as you pay for the rental in full with an Amex card that has the coverage. Each Amex card is different and not all rules apply to each one.
You also don't necessarily have to pay extra (the $25 premium coverage) to get the Amex coverage to step up as primary. Amex basic coverage (free) through certain Amex cards (you will want to check with them as it is specific to certain cards) also becomes primary if there is no primary. The premium coverage is a premium add-on that covers extras including more expensive vehicles.
Also, even when a premium card (i.e., Chase, Amex, Citi, etc). has CDW coverage. it does not necessarily cover every expense or aspect of a situation. There are certain exclusions and exceptions. Important to remember.
+1
All excellent points... you really (as in, really, really...) have to read the fine print. And for folks who don't carry a traditional auto policy (i.e. don't have a car) most credit card secondary coverages do indeed become primary. I would just add to check with the specific program to make sure the one you wish to rely on does... it's definitely not something you want to have to 'figure out' after the fact.
Unrelated note: Is anybody else having trouble geting Lithium notifications? I've not received an email notification of a new post or kudo since early this AM. It might just be me, though... Edit: Nevermind... confirmed with gdale6 there is a problem of some type with notifications.