My own experiences with H.D. Radio have lead me to the following conclusions:
1. H.D. Radio signals come in better when stationary, which is ironic given that the majority of H.D. Radio receivers are in automobiles.
2. An outdoor antenna designed specifically for the F.M. band can significantly reduce and in some cases even eliminate dropouts for H.D. signals - even if the receiver is far from the transmitter.
3. Per point 1, I wouldn't recommend the use of portable H.D. players, especially in areas located a fair distance from the transmitter.
4. It seems that H.D. Radio signals received in a NON line-of-sight manner will still come through although the reliability is rather poor, presumably due to ducting and skip phenomena. Again, per point 2 an outdoor F.M. antenna may allow for reception of non line-of-sight signals with good or even perfect reception.
5. Watch the signal-strength meter! I find that the signal comes in reliably as long as I have three bars or more (for a five-bar scale). Two bars works but causes occasional dropouts, whereas one bar is basically a "don't even bother" scenario.
I hope this helps.
