I think Darren's putting them in some driving lights so not a huge issue of failing when away from home as he'll still have his headlights.
Personally, I wouldn't bother with HID's in headlights for a number of reasons. The two main ones being, more attention from Police and having a ballast issue causing one or possibly two of your headlights to not work.
One of the other issues is, the majority of headlights are not designed to run a HID bulb and emit spurious light which is not pleasant for oncoming traffic or at times the driver of the vehicle with reflections off signs etc.