During my24hr race I decided to hire a helmet mounted light to go alongside my bar mounted one. I currently have an Exposure Maxx D light which is just totally fantastic. It belts out 1296 lumen's for 3 hours and is a totally sealed unit meaning no faffy cables. Even when used at a third of the power (about 800 lumen's) it is still bright and will last for 10 hours.
But bar mounted lights have their limitations. No matter how bright they might be. If you are on a twisty trail with switch backs you just can't see around the corners, you just end up looking into a vague dark area. This doesn't make for the fastest riding. A helmet mounted light is the solution. Step up the lovely Exposure Joystick.
Cheers for your contributions Guys and Gals. Great stuff!