The problem with a light on your helmet is that it flattens everything out - no shadows to show how deep that hole is, or how big that rock is. But they are great for spearing the typical zombie drivers who are always staring straight ahead without seeing. Wakes them right up when you zap 'em with the spot beam from the helmet light. This is one case where alien-abduction blue tint is perfect, really stands out from the orange streetlights and standard halogen headlights. Also great for lighting up corners when the bar-mount light is still pointing straight ahead.
Obligatory on-topic advice: I got a roll of velcro and used it almost like duct tape - ran it through the vents in my bicycle helmet to hold an LED flashlight on top my head. It makes an excellent supplement to my LED bar-mounted light by cateye, a triple 3 watt LED that works quite well for lighting up a road which has no street lights. The helmet light is for cornering and making sure those drivers in cross streets and driveways know I am coming.
Don't mess around with DealExtreme crappy lights when it comes to bicycle lighting. You want one that will not fall apart, go out, or change modes every time you hit a bump. Waterproof is also a great asset. Think of the light as a piece of safety equipment that will save your life, and that should make it easy to spend the money it takes to buy a high quality light that will not send you to the hospital or the morgue.