I have HD,I have the Directed HD add on unit. I used it coupled to my McIntoshMX4000 combo and it was pure bliss.
Well worth $100 for the box IMHO.
The HD part will fade and when it does fade back into regular FM you can get distortion. Which is what happens to HD unless your on an HD2 channel, then its either all or nothing. AM sounds like FM, FM moves to CD Quality without static, etc.
Again I would buy a stand alone unit because most units with HD built in are not that good of units. Furthermore the stand alones will work with any radio.either by FM modulation (antenna pass through) or by RCA inputs on the head unit.
http://www.crutchfield.com/App/Produ...60744000&tp=17
I paid $100ish for mine through E bay.