While I wouldn't consider it the "optimal design for receiving HD channels" the Gray-Hoverman does work well for receiving UHF stations (70% of US stations). It doesn't perform on VHF like some other designs, and I wouldn't recommend it for lower strength VHF stations. I've built two for my last two residences.
A reflector screen can be added 4 inches behind the elements to give it added gain in one direction. I didn't use a reflector since I'm getting stations from close to 180 degrees from each other. I use 1/4" copper tubing for the elements.