The ones that don't blind the hell out of me. I.E. none of these cheap ass plug in kits.
If you disagree try shining a million candlepower spotlight into your lights while driving down a 2 lane road. Maybe you understand now.
+1. I originally had an HIDguy kit in stock housing. After one night, I took them out and put the stock bulbs in. The light output looks brighter but visibility does not get better. It just seems like your visitiblity gets better because the light scatters in all directions so you can see stuff above and to the sides of where you should with proper headlight cutoff. Not to mention everyone in front of you gets blinded because of the improper light cutoff. This was 3 years ago.
I kept the HID's in a box and last year I bought a retrofit kit from the retrofit source for $120 and it came out great. It looks factory and the visibility is much better with a sharp cutoff so no one gets blinded. It looks very similar to the clay mockups they created for our cars during development. You can see them in the book Iron Fist, Lead Foot.