The "HD" acronym is thrown around a bit liberally. With optometrists now referring to a new set of glasses as, "High Definition," the term is rapidly heading the way of digital, as if anything that was and is digital is somehow better quality. It is a pleasure, however, to note that HD Radio does not do the acronym a disservice: HD Radio has been a significant improvement over AM- and FM-quality radio for people that enjoy radio.
I have been using the JVC KD-AHD39 for about two months now, and have nothing but positive things to say about its HD Radio function (indeed, the unit itself is great). The HD modulator - the electronic guts that tune in an HD station - works automatically. You visit your favorite local station via its FM equivalent, such as WBCN 104.1 in Boston, and if a High Definition digital signal is being "piggybacked" on the analog transmission, it automatically tunes it in, giving you CD-quality sound, or so the marketing folks claim.
CD Quality or Digital Quality?
In testing I have found that the quality varies wildly, however, though it is always significantly better than FM and easily equal to satellite radio. Basically, you tune your FM station and it instantly pops up, then it takes about 2 seconds to tune in the HD simulcast. The quality of the transmission depends on the broadcaster, just like television. The radio station mentioned above, WBCN, has a terrible HD1 station (more on this later) that sounds muffled. Several other stations have some hiss or white noise that picks up in the background. While this might seem like a complaint, FM pales in comparison. Stations never fade in and out and are less subject to interference. In most cases the quality easily outperforms a high-quality MP3.
If a station does "drop" from HD it reverts to its FM broadcast. As a listener this means you can continue to listen to your sports broadcast or favorite song without interruption, though it reverts to FM-quality. High Definition signals occasionally drop when signal strength fades at long distances from the tower.
More stations, more choices, better quality
The reason that WBCN HD1 is worth mentioning is that there is a WBCN HD2, an entirely different station with its own unique programming broadcast on the same channel. Best of all, most every HD2 station I have listened to has been commercial-free. Adding HD Radio doubled my content access while driving to and from work. Further, HD Radio allows you to view song title and artist, and some players will let you download the content to order music from iTunes (though I do not use iTunes and could not test this feature).
In short, there is no reason not to get HD Radio if you enjoy listening to the radio and have the option via your equipment. Once you have an HD Radio tuner, all of the content is free, it is significantly better quality, and there are more choices. Check the HD Radio web site for information regarding stations in your area.
Note: The particular car head unit used is overkill for most people unless you are outfitting your car with a full-blown system including multi-channel amplifiers and subwoofers. Most listeners will be perfectly happy with a lower-end unit.