Indeed HID are efficient as I did put it on my list, look at point no.2

However, HID are so fragile that they cannot take any shock, once drop on the ground is sure to render the light useless and malfunction. That was what happened to my HID and I have yet replace the bulb. And if you want to compare M6 to a HID (of any brand other than SF Beast) than I should say that M6 is build like a tank, it is design to be mounted on firearm to take the recoil shock, and were field tested.

So indeed HID is not a light to be use for rough situation. The only HID that can take that kind of shock is the Surefire Beast, Beast II and Surefire Hellfire. The Hellfire (HID) is design to be mounted on GPMG!!!
What is the point of 90mins runtime, and if you dropped your HID, you'll be out of light. And yes, it last very long provided if you use it properly, as you should know that HID isn't instant on/off. Inapproriate use will also cause the HID bulb to spoilt faster.