I use it mainly on international rentals just to make my life easier if anything goes pear-shaped (fortunately, nothing yet has). Note that if you enroll, all your AmEx cards are enrolled...if you want to go without, you'll have to use a non-AmEx card for the rental, otherwise it auto-bills with each rental.

It is primary insurance, and covers:
- Damage or theft, up to repair cost or car value, including loss of use
- Loss of personal property, up to $2,500 (signs of forced entry must be evident)
- SECONDARY injury benefit, up to $7,500
- Doesn't appear to be any limitation on exotics/expensive cars

It DOES NOT cover:
- Rentals in New Zealand, Australia, Ireland, Israel, Italy, Jamaica. All others are covered
- Damage to another vehicle, or injury to someone not in your vehicle
- Trucks (except pickups), cube vans, box trucks, mini-leases, modified vehicles, vehicles over 20 years old or not manufactured in 10 years, limos, off-road vehicles, mopeds, RVs, golf carts, campers, and trailers
- No mention of roadside assist, but Plat/Cent come with that - just call Premium Global Assist (not sure this actually applies outside the US)