I've read the fine print. On both my Gold Visa cards that offer this service, the insurance is the primary insurance and you must decline any other insurance offered to you by the rental company. In terms of damage, the coverage is 100%. Personal and medical coverage is also offered on top of that, but that's outside the scope of the rental insurance. The insurance is global, with the exclusion of combat zones and a few other bits not generally applicable to a vacation or business trip I'd be on.

Policies are underwritten by generally well-known insurance companies. Unfortunately the US credit card industry seems to be a lot different than the Canadian, so I have no idea how my coverage compares to products in other markets.