OK, on to further details as promised.
One downside of HID lighting is that there's more stuff. The old halogen lamp in your bike is just a lamp (BTW 'lamp' is the correct technical term for a light bulb). HID consists of the lamp, a ballast, and often a small transformer too, plus the wires to connect all these components.
Here's what it looks like:
The lamp is at upper left, inside a protective plastic cover which is removed before installation. The silver box at lower right is the ballast, and the black box next to it is the transformer. Obviously, you'll need somewhere to stow these components. The ballast in the picture is about 3.5" x 3.5" x .5", and the transformer is about the size of a matchbox. You will need one ballast per lamp, and one transformer per ballast if your ballasts require them (some have internal transformers).
On a nekkid airhead, it would probably be a good strategy to pull the headlight wire back from the light bucket and re-route it to somewhere under the fuel tank, where it can connect to the transformer, and then the wire from the ballast can be run out to the bucket. Bikes with fairings will probably be easier.
Now for some good news!
In the picture above you can just barely see a couple of blade terminals close to the base of the lamp. Those terminals are all you have to connect to the bike- just plug them into the socket that the halogen lamp plugged into...no modifications to the bike's wiring are required!
One other detail: HID lamps are like gymnasium lamps- there is a delay between the moment when power is applied and the moment when the lamp reaches full brightness. Luckily, for vehicular HID's that delay is only a second or two. What this means in real life is that for vehicles that use separate lamps for high and low beams, it's not much of a problem except for when the high beams are flashed to tell someone to get the hell out of your way or other signaling uses. The exception to this is the H4.
Halogen H4's have two filaments, each at a different distance from the back of the headlight reflector. There are H4-base HID's that use this change in focal distance to the reflector to make high and low beams. How do they do this, especially when they take a second to warm up? Easily- they have a small solenoid, controlled by the vehicle's high-beam circuit, that moves the lamp's gas-envelope a few milimetres to produce the required change in focal distance. Just like with the single-beam HID's, it is plug-n-play with the vehicle's electrical system.
Now to complicate things a bit.
When you shop for HID's, you will be presented with some options. As mentioned in my previous post, you can get HID lamps in all common H-type base styles. But there's another option you have to deal with when selecting your lamp, and that's
colour temperature. Basically, colour temperature refers to the colour of the light emitted by the lamp, expressed as a temperature (I'm not certain whether or not it is the actual temperature at which the gas in the lamp glows or some other type of measurement). The setup I used on my GS used a 6000 Kelvin lamp, which meant a nice very white colour of light. The merchant from whom I bought the kit also had lamps at other values, such as 4500-5000 K which were a little yellower, or 6500-7000 K which were a little bluer.
Here's a good shot of the colour of the light. In this picture the HID low-beam is operating, and so are a pair of 35W halogen Motolights:
Notice how yellow the halogens look compared to the HID.
You may also be presented with options for different ballasts. For example, my merchant had two different ballasts from which to choose, the main functional difference being that one was about 10% more efficient, which translated into more light output from the same lamp, but was also larger and costlier. I chose the smaller, lower-priced ballast and found the output of the lamp to be quite satisfactory, but then my bike's headlight reflector and lens were fairly well-designed too. Less-good headlight designs may warrant using the more efficient & powerful components. My result was a nice, clean, very white light that was great at night because the clean clarity of it really illuminated and defined everything upon which it fell, almost like a little piece of daylight in front of me. It was also great for daytime conspicuity.
Prices and selection continue to improve, and already a decent HID setup will pay for itself if it outlasts 4 or 5 halogens (or maybe less- I haven't checked prices in a couple of years). This technology continues to gain popularity, and BMW offer it on a few bikes in the current model-linep.