Someone already mentioned that the AMEX is primary, and other cards' insurance is secondary to your own coverage. This is a very important distinction for those (like myself) who have older cars and do not have any comprehensive/collision. In my case, I have no primary coverage whatsoever, since liability won't cover this. I'm not sure, but I suspect that normal secondary card coverage wouldn't pay anything because of that. I'd rather not take the chance. I figure the odds of my having an accident are magnitudes higher in some busy city that I'm unfamiliar with than they are at home. And in cities with limited parking, odds are decent you'll get dings in parking garages where cars are wedged in like sardines.
As far as walking around and getting them to sign off that there's no damage, this can be very, very difficult at the busy locations. I stick with Hertz because of this -- they cater mostly to business travelers and they won't nickel-and-dime because that will cost them business. If you read travel forums, you'll find they tend to be a lot pickier at the budget places that cater mostly to leisure travellers. I read particularly bad things about Enterprise. Hertz and Avis do cost more, but I think I'm less likely to have some bogus claim crop up with them.
I take many pictures before I leave, & I mark the heck out of the pre-rental form. And I do my best to get someone one to write 'no damage' and sign it when I return.
Starting Score: 703
