I've used TravelGuard numerous times, for international trips and ski trips, but never for
DVC. On two occasions, family illnesses forced us to cancel trips to take care of in/out-laws (MIL broke her leg, FIL had heart issues). The claims process was very quick and easy, and I highly recommend Travel Guard.
Now, whether it's worthwhile for a DVC trip depends greatly on you. The devil is in the details, and as the previous poster suggested you really have to assess your risk of cancellation. If you are a DVC owner, Travel Guard will only reimburse $5 per point for points lost, I believe, but of course your other travel costs may be much greater than your DVC points. You just have to look at it and weigh your risk/reward vs. the cost of the insurance.
It's also very important to know what any
trip insurance covers and what it doesn't, and especially what
causes are covered. You have to know what you're buying. In both of our cases, our losses were fully covered even though neither relative was going with us, because we had to cancel to take care of them.