The HD2/HD3 I mentioned refers to the multicast channels that some stations are now implementing. There's more info here
but in general, an FM station is either analog, analog + HD, or analog + HD + multicast channels. The first channel mirrors their analog, the second has another program and some multicasting stations have a third stream with yet another program. Often these are either broadcasts of some .com that is available from the internet, a mirror of an associated AM or FM station, or a news or comedy network. As posted here by another, some stations really butcher the audio they send out, either analog or digital. Not only is there compression to make music have more punch, but with digital, there's even audio more mangling introduced, both for MP3 and now also multicasting. Some stations are just unlistenable, whether analog or digital, some are fine for car or home background music or talk. IMO, the only advantages of HD Radio are clearer reception and more programming availability, not so much for high-end audio when multicasting. The better ones where I live sound about the same, and some better than distortion-free analog FM in my setup too. My experience with it is very positive.