Obviously look carefully at your specific booking for the terms of CDW and any applicable excess. They do vary by rental car provider and US state. Also look at what is not covered - typically windscreen/windows, tyres and roof may not be covered and there may or may not be a cap on your liability exposure for those items.

If you do decide to take top up rental insurance, typically any additional driver is only covered IF they are named on the primary rental agreement. The typical US additional driver (family member, business associate) that don't require naming on the rental agreement for the purposes of the rental company, will not be covered by top up insurance. So if you decide to buy top up, insist any additional driver must be named on the rental agreement.