Lots of good comments and I'll add a couple. One thing no one has discussed is cutoff. I have factory HIDs in my Dodge Challenger. In low beam they have a very sharp cutoff line, in other words they only shine so high. If you add HIDs to regular headlights the reflectors and lenses aren't designed for them and you don't have that cutoff and blind other drivers.
HIDs are great, they only shine down the road a little farther but shine much higher and wider on high beam and there are less dark spots.
I see the same issue around me, guys are putting HID lighting in their trucks that have conventional headlamps. HIDs are designed to be used with projector headlamps giving the projected lighting pattern with the cutoff avoiding blinding on coming drivers. Some HID lighting providers are offering dipped HIDs lamps to help prevent that but I don't these being used.
Many truck models have after-market headlamp housings available that offer projectors. If you chose HID lighting please consider using projector lamp to avoid blinking the motoring public.
One other issue is the Kelvin temperature of HID lighting, many believe the higher the Kelvin values the more light that they can make. Kelvin is color temperature, the maximum amount of light is at 4,300 kelvin, this is typically the temperature used by OEMs. LED lighting is somewhere in the 5,000 to 6,000 temperature range. Halogen is around 3,400 Kelvin and noon daylight is around 5,000 Kelvin.
The lower the temperature of the light, the more yellow it is, as the temperature increases, the light changes from yellow, to white to blue. If you have older eyes (like I do) and often drive country roads, in the dark, and on rainy mornings, you will be pleased with 4,300 to 5,000 Kelvin temperature lights. There are many color charts on the Internet if you Google for them.