Good luck finding major motion picture theatrical HDDVD movies. They don't even produce them any more. Your best bet is on ebay. At least you can use it as a nice upconverting 1080p standard dvd player it you have a cheapass lcd tv with a crappy scaler.
If it exists on HDDVD then get BBC's Planet Earth documentaries. Seeing wild life in action, such as the seasonal journey's of the elephant across or a saharah or wild mountain lion in action take down a mountain goat in 1080p is the very best reason to get a Bluray.