Lee,
After having my share of the fancy H4 bulbs quit working from the excessive vibration motorcycles can induce, I can see your concern. What happens to the halogen bulbs is the filament gets stretched and/or brittle while current is running through them, creating metal fatigue. They can stand only so much.
With the HID, you have two small tungstun electrodes inside a gas capsule filled with Xenon/Krypton gas. Here is what the electric arc looks like in action:
http://www.sylvania.com/xenarc/images/hidarc.gif
Their size is so small and supported by the surrounding glass housing. There is nothing to fatigue with vibration. When I was testing the PIAA's, I turned them on and then hit the housing with a hammer to simulate the torture they could encounter. They did not fail. So far they seem to be very stable, even down the washboard surface known as the I-405 freeway in L.A. Check out the picture on my page showing the difference between the two bulbs.
There is nothing special about these bulbs. They are the same Philips bulbs you would use in a car. From what I have read, Philips makes the best HID element around. Different companies buy the elements and then install them in the different mounting bases for the multiple bulb types. The glass element on my H4s look exactly like the one for the H3s. The mounting is the only physical difference.
One thing to be aware of is the Kelvin rating of the bulb. They use a mixture of different gases to get the desired light output. For a reference, natural sunlight has a rating of 5700K. The higher you go in the Kelvin rating the more blue the light will appear. If you are looking at the different shades, the two best for the most light produced are 5200K and 6000K. The 6000K will give you a very slight blue tint to the light produced. I have seen one set for auction on ebay, boasting a 12000K rating. For all practical purposes, it will not produce enough usable for night time riding. It will be way too blue and not appear to be very bright. Also, you should never buy a bulb that had a colored coating on them. The coating acts like a filter, blocking the total amount of light produced, similar to the coating on halogen bulbs.
A fellow rider has had a 6000K HID low beam in his 2000 BMW K1200RS for almost a year with no problems at all. When he installed the light, we would follow behind him, comparing the difference. He could produce more light with his low beam than several of us combined.
If the bulb lasts two years, it will more than pay for the multiple pairs of Super/Hyper/Ultra/Xtreme white halogen bulbs I used to buy. I was burning out one of these "higher output" bulbs every four months or so.
Enjoy,
Randal