I've heard HD makes AM sound like FM, but only if you can lock it in consistently. I have the HD Radio adapter and app for my iPhone, and, when you can hear it, it's pretty cool, but the adapter only works on FM. I suspect the lower power requirement makes AM even harder to get than it is on FM.
Last weekend, I was in Kansas City, and I could hear some of the FM translators that relayed HD channel multicasts farther than I could hear the HD signal!
No - I have HD radios. It makes AM sound like medium bit rate streaming, in other words distorted and annoying - creating listener fatigue. I can get HD lock on WOAI when they run HD - quite a feat from Houston. It only takes a three foot loop, not your standard AM antenna these days, but I purposely force the receiver into analog on WOAI because the digital is so fatiguing to listen to. And analog WOAI even from 200 miles is completely clean and static free. Again quite a feat because HD saps a lot of the strength out of an AM station - as WBAP discovered in Ft. Worth and dumped it years ago. I guess their large, legendary groundwave and skywave signal was more important than it is to WOAI. But - I would think - WOAI would care about how HD makes their signal weaker over San Antonio and would care about building penetration.