You know that compression format that they use to store your voice mail digitally, that you can hear? It's like that, only not quite that bad.
That said, I'd still go for it, but for entirely different reasons -- content. They do have some pretty cool stuff on them. WTOP has this thing where they repeat the weather and the traffic all the time rather than every ten minutes. There actually have been times when 9 minutes from now was not soon enough.
So the thing to do would be to find out if any of your radio stations have any HD sister stations. Which you did, and they don't. So go iPod navigation.