Some sources of information follow. Budget Light Forum is where the action is. Candlepower Forums have fizzled out for some reason.
When underground, my primary light source is always a headlamp, mounted on my helmet. But, I always carry a flashlight, almost always on a lanyard around my neck. This way I can drop the light at any time and use both hands whenever I want to. Hanging it on my neck means that I have a handy light for close inspection of whatever I want to inspect.
These days the only thing anyone should consider is an LED light of some sort, both for headlamps and flashlights. Some people complain that the light from these is "too blue". You can either just get used to it, or select a light with a warm emitter. I much prefer warm emitters and won't buy anything else. There is a slight penalty in lumens, but these days lights are so bright that this simply doesn't matter.
The overwhelming advantage of LED lights is their efficiency. Because they use so little power (because they aren't wasting most of the power the consume producing useless heat), batteries in an LED light seem to last almost forever.
LED flashlights and headlamps have become a hot item for consumers. New products and new LED technologies seem to roll out every year, so it is very hard to stay on top of things. Needless to say, you can buy a much better light these days than you could only a few years ago and there is no telling how long this will continue. So if you loose or damage your favorite light, it just means you have an excuse to replace it with something much better. Consider the implications of this if you are considering buying a several year old model on clearance. It will probably be a fine light, but you could get something a lot better if you bought a new model. I have heard good things about both the Fenix and Nitecore lights (sold by "4 sevens"). They also have their "quark" house brand, which is probably just fine.
Tom's Old Mine Info / tom@mmto.org