I use these things all the time.
I have the following:
Petzl Zoom (Old School) with normal and Halogen bulb.
Petzl Zipka (New School)
and now the good news. Do not buy any other head torch than this one:
AlpKit Gamma (go google)
Sure these guys work from a shed, but they ship next day and the stuff is very cheap and the quality better than Petzl. The only downside is no rechargeable batteries.
I often use the Zipka for a normal trail plod as the light is enough and the batteries last for ages (months). If it is tempo or intervals I use the Gamma. Sometimes run with the zipka on my head and the Gamma in hand to give extra light when needed.
Why the Gamma. well for £15 it has the same light output as the £60 Petzl. Plus it is a better light for running as they aren't so mass market they do not have to compete for the longest beam and therefore focus on the most useful beam. A good pool of light is better than spot in the distance when running.
One click, bright, dim, flash, off
Press and hold and it switches to a much lower wattage led switching between white, green, red, red flashing and off. This gives days of battery life from 3AAA held on the head. This balances the weight of the light so it sits nicely on the head.
On the battery pack is another switch for the rear bar LED. Red, Red Flashing and off. Great to be seen from behind, the motorists should never miss now.
This is the best head torch bar none for running. I have two. Big light when required, small light for battery life, rear light for visibility.
Actually I lied, the best head torch has got to be the Petzl Ultra but isn't really for a trail plod, more like Moutain Rescue (£300+). The Silvas are very nice if you want a bin lid on your head.