Generally speaking, the risk of you having an accident in your rental car is equal to the risk of having an accident in your own car. So if you drive everyday at home not worrying about having to use your insurance if something happens, why would you worry about it when you're on vacation? (Yes, I know you're familiarity with the car itself and the roads you're using are a bit less in the rental car but the odds are still quite minimal.) Nearly all travel experts feel that buying Collision Damage Waiver is a very expensive error.*
BobK/Orlando
*this advice applies to drivers residing in the United States that have collision coverage on their home automobile policy. Always verify that you're covered driving any non-owned auto, particularly a rental car. Drivers from Canada are often covered with their policy back home as well although one of the posters above apparently was not. Drivers from Europe will nearly always need a package price that includes CDW since their policy back home would not cover driving in the United States.