I haven't had any problems with HD radio on my Toyota Prius and regularly get the Emmis station WFNI with no problems 60 miles away. WBAA in West Lafayette, a much lower power station, comes in most of the time at 50 miles away. I did have trouble myself in the Louisville area with HD this summer so it may be the terrain.
Someone wrote that Ford is putting HD radios in all of their newer cars and I know Toyota is, also. I cannot fault the signals that I receive. Many broadcast engineers hate HD because of the interference but WBBM in Chicago causes no interference on 800 Khz. from what I have heard from a DX'er living in Chicago picking up a station in downstate Casey, Illinois.
From my prospective, HD radio really makes a big difference in the signal listen-ability. As with everything there are technical problems to overcome and we could debate the merits of this technology for awhile. However, one thing is certain. The AM band is dying a slow death without something to stimulate it and I believe HD radio is the solution technical problems or not.