The performance of HD varies greatly depending on a lot of factors. I've had really good luck with HD in some markets, while others have been dogged by issues. I probably have a worse experience than those with HD built in to their cars because I'm just using the little old Insignia portable with the antenna wire strung up around my car's rearview mirror — hardly an ideal setup, especially for weak FM signals.
As an overall market, the best experiences I had with HD were in St. Louis. I seemed to have dropout-free reception around the entire I-255/I-270 loop, and reasonable reception all the way out west to somewhere around Washington. I think that was around 50 miles, and also the point where the analog signals started to degrade a bit.
I only got about 20-25 miles of good HD reception off most of the stations in Memphis for some reason. WMFS-FM, the local FM sports station, is a class A and their HD only worked inside the I-240 beltway for me. That was only about 5-7 miles from the TX site!
Mississippi's public radio network, MPB Think Radio, is all-HD and they are all pretty solid performers. The station based in Oxford in particular was a standout, with reception from Memphis' southern suburbs all the way south past Grenada possible along I-55, with very few interruptions. Their Biloxi repeater's HD reaches Mobile's western suburbs somewhat reliably, and I think that's again around 50 miles from the TX site. I'm sure with a decent car HD radio it would be a viable option that far out.
Here in my home market of Mobile & Pensacola, it's nice and flat so the HD signals can be received for a good 50+ miles, but they are very spotty when moving for me since the little Insignia is prone to RF overload. None of them are solid on the go except WKSJ, which is running higher power and has a real observable coverage difference compared to the lower powered HD stations. All but one of the HD stations in this market are the full 100 kW, often at 1300-1700 feet HAAT. Pensacola's pubcaster WUWF also probably operates at higher HD power, as it is the only solid HD one can receive in that city.
Up in Montgomery, Alabama, the HD signals don't have much reach (Montgomery is in a bit of a bowl, geographically, so that limits lower powered signals from really getting out) but they are absolutely solid in-town. Not that it matters, as most of the HD content is repeated on translators. There's 12 active translators in the market and four repeat HD subchannels. The translators have about the same range as the HD signals, since they all originate from lower-powered stations. Alabama State's WVAS is 100 kW from a stick right in the city and their HD doesn't get out any better than the smaller stations. Maybe 10 miles max before the dropouts get to be a problem.
Birmingham is the worst HD market I've heard. I don't know if it's the hilly, iron-rich mountainous terrain or the HD stations there are just poorly sited, but I have never had solid reception of any of them for more than 30 seconds, no matter which station at what power. Maybe it's RF overload on the little Insignia's part. But for whatever reason, I couldn't hold an HD station there for a minute to save my life, even when within eyesight of the main TX towers. Not that it matters much: Like Montgomery, most of what's on the subchannels is repeated on translators, some running 250 watts from well over 900 feet HAAT, so they're like little Class A's on their own. Of the 12 (yes, twelve) active translators in the metro area, 7 relay HD subchannels.
Another market I recall being poor for HD was Albuquerque. You'd think with everything being beamed down from high atop Sandia it'd be ideal since the entire metro is line of sight to the mountain, but I had nothing but trouble there. One oddity that stood out was (IIRC) KZRR at 94.1. I had great HD reception EAST of Sandia in the rural middle of nowhere for a good 50 miles, but once I crossed into the city, it was gone. A week prior, I logged that exact same station, in full HD, in central Mississippi during intense skip. So I can honestly say I got good HD from them from Mississippi but bupkis in ABQ itself, lol.