I bought a Kensun HID kit off Amazon. Yeah, one for a car. My reason for that was that'd I'd have spare parts down the line if needed (two lights and ballasts with a car kit).
Hacking the wiring down to one light is not hard.
My kit is a 35w kit. That saves me some power on my alternator over the stock halogen.
I also got a kit in the color temperature of 5000K. Most of them time, this color temperature is considered the most 'white' looking. I didn't want my light having that terrible HID Blue look.
The kit I am using is a Hi/Lo "Bi" model. Pretty much the bulb is attached to an electromagnet and when you switch to your high beam it pulls the entire bulb further back into the housing and that causes the light to focus differently through a little window. Something like that anyway.
I've been using it for well over a year with zero problems. And I mounted the ballast all exposed on the exterior of my Savage. Its pitted to heck, but still fires up every time no problem! =D
A side note on some of those LED lights before I forget: there are reports that the light is so focused down ahead of you, that your headlight essentially appears turned off when viewed from the side. If I'm driving around at night down the road, I want to make sure some guy merging at an intersection sees me! You can get an idea what I'm saying here:
http://www.hdforums.com/forum/touring-models/741065-truck-lite-phase-7-led-he... check out the side view shot. So just be careful!
Anyway, back to the HID related stuff...
I was not able to cram the ballast or voltage step up box into my stock headlight. The bulb and relay box was a pretty tight fit but I got them in there. Whatever kit you buy you may have better results stuffing.
Most recently (today actually) I switched out my headlight to a larger 7" one. This has made a dramatic difference in everything forward illumination wise.
For one, I was able to cram all the HID parts into the headlight where they are totally protected from the elements. And no more crap zip tied around my gas tank and forks! Yay!
For second, the focusing pattern is much better. So much more of my light gets to a useful place on the road.
For third, the HID's Hi/Lo function I found to be pretty useless on the stock headlamp. Whenever I had the headlight pointed so that the light made moderate sense on the low setting, the high setting wouldn't even touch the road. First run today however seems to have good results on low and high. Everything is just thrown a better direction. Still, only first run, so we'll see what I think in a few days. Looks like win though so far.
Lastly, when I made the move to the "Bi" HID and its magnet movement science junk, I got the 'bonus' of my actual light bulb bouncing around with the vibration of the engine. This is because when the bulb is not being pulled by the magnet, it is just kind'a hanging loose a little. Going down the road, I get the 'modulated' look at different RPMs. Safety feature? haha!
Your brand of bulb may give you different results.
Bonus tip I saw elsewhere on this form! Tie your HID light's power source into a relay that is normally closed. Then tie the relay's signal wires into your decompression solenoid's power source. That way, when you start your motorcycle, the relay goes open and turns off your headlight. This will free up a bunch of amps that will help your starter turn over better! After your decompression solenoid is done with its business, the relay goes back to being closed and your headlight gets back to being bright. (I was able to stuff this extra relay into my 7" headlight bucket as well).
Hope some of this has been useful for ya!