i'm personally a sucker for HD content without the fee. For example, OTA HDTV is great...and FREE!
I have been interested in HD radio for this reason. A lot of the radio stations I listen to are also broadcast in HD, both FM and AM. If you listen to a lot of radio that is also available in HD, it sounds like a great upgrade.
It's all digital so i'm sure it will be a big sound improvement. It's supposed to offer near-CD FM quality, and FM quality for AM.
I don't see a lot of options in receivers though. And from what i see they are kind of expensive.
Are you looking for car use mainly, or even home?
Is there a portable receiver that can be used for both applications? I'd be interested in something like that myself.