I have an Exposure Toro as a bar light so I understand the praise for the Joystick but I've gone a different route for a helmet light. I only use my helmet light on descents (when I need the extra illumination either because it's techy or I'm going fast). With that in mind I ended up buying a Klarus Mi7 which is a tiny 14500 cell powered torch. It's amazingly bright for something so small and so light that I don't even notice the extra weight on my lid (another reason for going for this light). I mount the light on a GoPro mount on my helmet and a universal GoPro torch mount. The battery only lasts about an hour but as I mentioned I only use it while descending so that's usually long enough. If I do need longer life the 14500 cells are very small and light so it's easy to take a couple of spares. The light will also run on a AA battery (albeit it's not as bright as the Li-Ion) which might be useful on an extended trip.

Maxx-d & joystick combo here