The question You should be asking is, are the HID kits legal?
Short answer is no.
If your car came with HID lights equipped on that exact car, then it is legal to drive with them.
If you do an after market HID kit ( both the cheap PnP kits from DDM and other companies, or do a Retro fit of you headlights using an actual HID projector from another car) then it is NOT legal.
Here is just one reference for you to review.
http://fmvss108.tripod.com/nhtsa_legal_interpretations.htm
Granted it isn't the law, but you get an idea as to how they view things.
Halogen headlights are designed for that type of bulb. The light emission point of the bulb and the shaping of the halogen reflector around the bulb are designed for a relatively low intensity of light.
The halogen lights we have on our car utilize a projector, similar to appearance to the ones you see for OEM HID lighting systems, they however are still designed for a halogen bulb and it's lower intensity light discharge.
HID projectors are designed with a safety feature to prevent the high amount of light being projected from glaring on comming traffic. When you hear about a cut-off this is what they are talking about. There is more to it than that but you can get the idea.
yes our Halogen projectors have a cut off feature, but it is not the same.
Will you get pulled over for putting a PnP kit in your car? maybe, maybe no. police do not know what cars have what lighting options, but keep in mind cops do drive other cars and may own the exact model you are driving and WILL know about that car.
having an OEM projector headlight will make it more difficult for a police officer to know if you are legal or not.
As for retrofitting another cars OEM projector into yours, probably the best and safest way to get away with non Equipped HID lighting in your car. Plus, the performance you get is much greater than any PnP kit you will buy.
They are expensive though. But you get what you pay for.
Costs may go down if a group buy is done.
I am looking at this option. Doing a quad FX set up or a hybrid FX and S2K deal.
Going quad or dual will lose your DRLs though.
If you've never heard of this option look at these sites.
Parts
http://www.theretrofitsource.com/
How to
http://www.hidplanet.com/forums/
Retrofitters (will do the work for you)
http://hidillusionz.blogspot.com/
http://www.lightwerkz.net/
http://www.customlightz.com/
Probably the most legal way to do it, is to buy replacement OEM HID lights from the dealer.
I'm not sure how the wiring differs. Not something I've fully researched.
Also, with HIDs, the bulbs color shift over time. if used long enough a 4300k bulb can look like a 9000k bulb, but that would take a VERY long time.
If your lights look too blue you will be pulled over and have the light tested to see if is a blue bulb or a HID bulb with a blue appearance. There is a difference. A white piece of paper will turn blue with a blue bulb, will will look more white with the hid bulb.
That being said 4300-5500k gives you the most useable light output. yeah, you might not look like you belong on the fast and the furious, but you'll be safe.
My civic had SC430 projectors in my retro fit with 4300 bulbs. The light was nearly white, and when you looked at the car, it had a blue/purple flicker.