I was able to check out some HD Radio broadcasts, and if you haven't had an opportunity to hear your favorite radio station in HD, then you're missing out.
This is how it works. Existing radio stations, both FM and AM, will have to spend a little money up front to upgrade their broadcast facilities to support the HD format. Once the equipment is in place, a digital signal is encoded into the analog broadcast and decoded by the user's HD radio. So stations can then multicast both the HD and analog signals on their existing frequency.
Another great feature that anyone who's car stereo is capable of decoding existing RDS signals (that will tell you the radio station, artist and song currently playing) is the increased bandwidth for that sort of data. The demo they were showing also included a weather report, news feed, and stock ticket, all available through the radio broadcast.
Currently at hdradio.com, there are just over 600 stations currently listed. Although they are all FM stations at this point, I was assured that this would work for AM as well, which would make listening to sports on the radio (which are usually broadcast on an AM station) much more dynamic to listen to. And yes, after going to hddice.com (they make HD Radio for cars) and checking out the listings there for my state, there were indeed AM radio stations on their list.
Of the remaining 12,500 or so radio stations in the US, 3000 are committed to converting to HD Radio, so if the stations where you are haven't converted yet, there's a good chance they will soon.
So is it worth it? Well, if you're buying a BMW, Mini Cooper, Toyota, Lexus, or Scion, you can just get one from Dice Electronics installed from the factory (the major American car makers haven't hopped on board just yet). And the Dice models also come with an iPod interface, so you can have AM, FM, HD, and your own personal music collection all available through a single in-dash unit. The average home unit appears to be running around $300 right now.
If you've already invested in satellite radio, then HD Radio may not be for you, unless you don't mind the initial cost for a receiver. If you currently do not have satellite radio, but are thinking about it, I would definitely recommend checking out HD Radio. The initial cost for a receiver is comparable, and as the HD Radio guys were quick to point out, it sounds just as good or better, and there is no monthly fee.