Good or bad, we start from there.
Standard 108 says that you can not put a HID bulb in an headlight which was meant for a halogen housing. One reflector is made for a thin strip of tungsten, and the other an arc tube which is round. The modern free form reflector/projector is designed to a tight enough standard that even if the arc tube is in the exact same spot as filament bulb, there will be stray light, hot spots etc... That is their reasoning, and we do not debate that because they make the laws.
What this means for you is that HID Kits are a black market. DOT will shut down any US importer of HID kits large enough to be worth the effort. They have ruled repeatedly that "off road" or "show only" kits are still illegal, you MUST have a valid "on road" use in order to legally import a product for US vehicles.
What ends up happening is you have literally thousands of different manufacturers producing kits for a limited time, changing names, brands products etc... may be the same company, may not be, but that limits your recourse if the kit goes bad etc... typically 6 months is the limit you will see a new brand on the market.
The challenge that these small manufacturers have are 2 fold, and will explain why a lot of people do not reccomend them.
1) Electrical ballast
2) HID bulb
1) Electrical
The HID ballast is a counterpart to the bulb, and must match it's electrical requirements exactly to the mixture of gas/salt in the bulb. The requirement is to turn 12volts DC into both an explosive charge at 24,000 volts momentarily, and then match the steady state 85 volts AC that the 35 watt capsules require to stay lit. This requires high frequency switching transistors, and makes a lot of heat and is very taxing. Smaller manufacturers may skimp on components inside the ballast, may not have water proof enclosures, or simply have a bad design which doesn't last.
Here is what a ballast must do electrically to match an HID bulb:
HID kits range in quality from OEM standards (good), to not even working properly out of the box (bad). The most common problems seen are flickering, high pitched whine, or overheat and cutting off.
2) Bulb
HID bulbs are a variation on metal halide bulbs which are commonly used in warehouses etc... They have a mixture of xenon, mercury, and other "metal halide" elements. The manufacturers must make a cocktail of salts/gases which will last a long time, produce a uniform color over their life spam (called color shift), and ideally produce a good color spectrum.
All HID bulbs shift to blue, and become dimmer as they age, this becomes exacerbated by smaller companies with no one to monitor their quality. You the consumer 1 year down the road have 0 recourse with them. Where as OEM consumers like car manufacturers can sue and/or not do business with them if they are having a lot of problems over the life of the bulbs which they have a warranty to keep.
Typical problems with after market HID kit bulbs include uneven brightness and color shifting of bulbs resulting in mismatched headlights. Not to mention bulbs failing at different times.
Here is a picture of a 5000k 400w HID bulb where you can clearly see the salt mixture.
The point of this discussion above is to show that smaller manufacturers have lower standards because it is a black market, and OEM manufacturers have higher standards. It's an economic situation which should explain people's preference for OEM equipment.
Besides quality standards, OEM components adhere to regulations on color temperature and spectrum. The only color which is legal is a nominal 4000k sometimes called 4300k or 4100k. As HID bulbs age, there is a 5000k bulb sold which is called "color match". OEM branded 6000k bulbs may be purchased, but never come on a new vehicle.
Color Temperature, Color Rendering, and the Blue Effect
HID kits are affected by their perceived color, their quality of light, and obviously the bluer the bulb is. OEM stay to one standard, and Aftermarket can do whatever they want. These are usually not discussed, and there are direct trade offs. The bluer the bulb, the less light you get because the salts which are more efficient are in the red/yellow spectrum. Mercury which is the blue part of HID bulbs is very inefficient. Sodium which burns red on the other hand is very efficient. The Scandium/Sodium (na/sc) is the red part in the picture above. Below you see how each salt has an effect on the ignition of the bulb (blue spark at start is mercury) and once it heats up to a certain temp other salts can ignite as well.
Aftermarket manufacturers make tradeoffs to give you the consumer a purple bulb, but which has less light, and poor color. OEMs must stick with one standard.
Probably the most commonly misunderstood concept in lighting, Color Temperature is a relative scale of appearance of color. The problem is that color temperature is designed for astronomy, measuring of a sun which has an equal distribution of color in it's range from UV 400nm to 800nm Infrared. Example here:
As you go higher in color temperature you shift to more blue, and less red basically, hence people associate a blue color with higher and red with lower. Generally correct, here are some more graphics for explanation:
Do not take these as representative of
REAL... because the trick with all HID lighting is that they are not a smooth distribution of light. Each HID bulb has peaks, and because it is not smooth, you could end up having an 8000k bulb which is normally understood to be blue, be red. Color temperature is only an approximation, so pretty much anything goes, and there are no real good ways to tell precisely what it will look like.
So because there are spikes, some colors do not display correctly, this has a name, Color Rendering Index or CRI. On a scale of 0-100, 100 is a smooth equal distribution of color, and 0 is monochromatic like a colored LED. Typical Car HIDs are in the 65% range CRI. They do not make a higher color rendering bulb... yet.
Here are two pics to illustrate Color Rendering Index which I feel may be the most important if not misunderstood part of lighting and HID kits.
This pertains to HID PNP kits because CRI and luminous efficiency are mutually exclusive. Meaning you can either have a high CRI or high luminous efficiency (output). You can not have both. A High CRI requires a complex mix of salts and is expensive, you can rest assured aftermarket PNP manufacturers do not use these. One way to compensate for this is to use a higher mix of mercury which will shift the color to the blue end which makes it appear brighter.
Everyone loves blue, it's nothing to be ashamed of, but in lighting you need to be aware of it's effects. The eye is nearly twice as sensitive to blue light as green or red, that means equal power blue and red makes it seem blue more powerful. This is terribly over simplifying it but it will suffice for our topic, the truth can be found here:
http://www.handprint.com/HP/WCL/color1.html
The problem with that is that the blue rods/cones are in the outside of the retina, not the center, so it makes it difficult to focus on. Try focusing on a pure blue light in a dark room, bob your head back and forth you will see what I mean, it takes time for the light to "settle".
So blue light is bad for lighting the roads because it makes it difficult to focus, this accounts for most people's bad impression of HID kits in the rain etc... where lower visibility is already an issue. But the blue spectrum also is less efficient inside the bulb, it makes less light than a bulb with say more red... as can be seen here, sodium is red, mercury is blue, and metal halide is somewhere in between.
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