Echoing what David has said above: Over the last few years on this board, I've noticed that a significant number of travelers (note that I don't say many, or most, or all, but a noticeable number) who
1) have had an "incident" (and actually have had to see how good their CDW coverage),
and
2) who used their credit card coverage for CDW,
have, in fact, reported hassles and/or problems.
In contrast, travelers who have paid the extra (a significant cost) for the CDW and/or super CDW from the rental company, have not reported hassles.
You never know how good your CDW coverage is until you have to use it.
The CDW coverage offered by the rental companies is a significant cost. There is some evidence in these pages, going back a couple of years, that it may be worth it.
Another alternative to investigate: Travel Guard sells renter's collision insurance at affordable rates ($9/day plus a one-time $3 service fee covers you up to $35,000, $250 deductible, tel. 800-826-4919). It's valid throughout Europe, except in Italy and the Republic of Ireland.
http://www.travelguard.com/
If your car-rental company doesn't honor this coverage, and you have to buy other coverage to replace it, Travel Guard will refund your money. Oddly, residents of Washington State aren't allowed to buy this coverage.