You got to be careful because there are a lot of fake HID bulbs and kits on the market. The tip off is if they are too cheap or don't have a ballast - a true HID will always have a ballast, and is unlikely to cost much less than $100 (though prices are coming down steadily).
The problem with HIDs, for use on older MCs at least, is that its hard to get a low and high beam out of them, for machines which originally had a single high-low beam bulb. The kits tend to handle this by either using a halogen for the low beam, or some even have a little solenoid that moves the bulb up and down or in and out to approximate a low and high beam. All which would be hard to squeeze in a Miller shell!
If someone ever solves the hi-low problem, these would be terrific lights for older bikes, since a 10 or 20 watt bulb could give a totally adquate beam, and still run off an E3L with power to spare.