Okay guys, I'm a convert. My experience with the kit leads me to tell you -- don't judge the kit until you've seen one installed.
Look further up in the thread for my opinion on HID kits, informed by having seen many of them on the road, usually awful purple kits badly installed in some kid's riced-out Civic. But at $35, I couldn't resist. So I bought a kit (4000K) and installed it. The install took about five minutes -- I cheated and used the existing headlight wires to power it. The bulb comes with a shadow mask that restricts the light to the top half of the reflector. This has the effect of imposing a pretty good cutoff line in the resulting light pattern when used with an H4 reflector, although it must reduce the total brightness some. The resulting pattern looks very good; although technically not legal for on-road use (since the pattern does not include the required left side cutoff), it does a very good job of minimizing glare to oncoming drivers. I was pretty worried about that, since my biggest pet peeve with HID is the glare problem. So far, no flashed headlamps at me, and the sharp cutoff line gives me a great deal of confidence in knowing where I'm throwing light around.
So, while not as optimal (and probably not as bright) as a proper projector housing, it throws a good pattern with a minimal amount of glare. And holy lord is it ever bright on the ground. I will never go back to a standard headlight bulb.
One more thing -- I bought the 4500K kit, and even that warm color temperature is a little blue for me. I wouldn't advocate the 5000K and up kits unless you really want a modern, bluish look for your old bike. I just wanted a much brighter headlamp with less power consumption.