I've got the handlebar mounted ARC light from Light and Motion. I wrote a lot about it on the commuting forum. The heading of the thread is HID lights, a review, or something similar.
That was a couple of months ago. I added a few posts to the thread on the commuting forum with more information. I've since used it regularly in a wide range of conditions.
It's the best bike light I've ever used, or seen. It's much brighter than any halogen system I've seen. It's much brighter, for example, than 20W or even 30W halogen lamps. The light permits full daytime speeds in the dead of night, and I never, every worry about being seen. Again, see the other thread.
The battery and lamp are very light. The lamp's design is also very well thought out. The lamp rotates vertically and horizontally. The on/off switch can't be turned on accidentally, the on/off button blinks in lower power mode, and the button blinks a warning with about 10 minutes life left on the battery. The lamp will shut itself down when the battery approaches completely kaput.
The charger completely refreshes a spent battery in about 3.5 hours, a very nice feature.
In short, the light provides blazing light, light weight, solid and intelligent mounting, intelligent design, and short charging times. Assuming the lamp last a couple of years, I just don't think bike lights can be improved without some new technological breakthrough.
The obvious drawback is the price. The light was about $350 when I bought it, and I don't think you can get it for less than $380 now. That's a lot of money, to be sure. You can buy good halogen lights for less than half the price.
If you ride a lot at night in dark places, though, the light is worth it. I was leery about spending so much, but now that I've had the light I think it's one of the best purchases I've made on bike stuff. It's significantly safer than my old halogen system, I think, from both see and be seen perspectives. I enjoy riding at night a lot more now.
I'd buy another one without hesitation if mine were stolen or smashed up.
For my money, the handlebar mount is preferable. The shadows are better than from a helmet light. You can also ride without a helmet, if you want to do that.