Depends who you ask. You ask the folks on HIDplanet, and anything HID in a reflector housing is a big no. Unless, of course, you have a car that came with reflector housing HIDs (the Navi), and even then, I'm sure plenty of them won't like them for the lack of a sharp cutoff.
Us second gen guys are lucky that we use a single beam bulb. The main reflector is meant only for low beam, so sticking an HID bulb in there isn't as bad as, say, the 1st or 3rd gens, which use one bulb for high and low. With that style, a small change from the focal point can be the difference between high and low beam output. Of course, I'm not saying us 2nd gen folk aren't blinding oncoming traffic, just that I'm sure it's to a lesser extent than 1st or 3rd gen Expys.
If you have a 1st gen, it'll help to get clear headlights as well, since the Fresnel lights will just disperse a lot of the light. The optics aren't as good on the aftermarkets, but that won't matter if you retrofit projectors. Just be sure to seal them up well.
I've heard from people that my lights are bright though not blinding. I still do pull up to the occasional car where my lights shine right into their side mirrors, and you see them hold out their hand, but then again that also happened when I had halogens. And it still would probably happen if I had a retrofit. It's just a matter of our trucks being taller than cars. I do believe I've seen some hate for SUVs on that forum for that reason (also that SUVs and truck drivers are the ones who tend to throw kits blue kits in all their lights.)
I used to have HIDs in my fogs, but all I was doing was blinding oncoming traffic. Those don't really have a shield, so they just splatter light everywhere. I now have some halogens in there, and only turn them on when there is actual fog.
I did lower my headlight aiming a little bit to try to reduce some of the glare, though that's not perfect. I also have 4300k since it gives you the most light as well as not being that obnoxious blue color.
Do some reading on HIDplanet and F150online (the F150 headlights are pretty similar to ours) for some how tos and a parts list.