Headlamp reviews note that the brightest available light isn't always the most useful. Lower light levels are better for reading (especially for reading in bed or in a tent without disturbing a partner), and also make batteries last longer. So the best headlamps have adjustable brightness levels. Most headlamps also provide a strobe mode. This flashing light can be seen for long distances, which is useful for emergencies.
Some manufacturers rate the maximum brightness of their headlamps in lumens. (To give a rough idea of lumens, a MagLite 2D flashlight averages about 36 lumens.) But manufacturers calculate lumens in different ways, and headlamp reviews say it's better to compare light output by the amount of lux at a certain distance. This gives a more realistic comparison of the light cast though it favors headlamps that cast a narrower beam.
The simplest way to understand lux is to compare it with moonlight; one lux is four times brighter than a full moon on a clear night. It's crucial to compare headlamp lux measurements taken at the same distance. FlashlightReviews.com takes the measurement at one meter from the headlamp, while REI.com uses two meters because in actual use, that's the average distance between a headlamp and the path just ahead.
Reviews emphasize that different headlamp designs have different strengths. At this point, no headlamp is perfect in all respects -- if it "does everything" it does so at the cost of weight or battery life. The main difference lies with the choice of LED -- which for most headlamps, largely determines the beam width.
Headlamps that use a single bright LED -- 1-watt, 3-watt or even 5-watt -- provide a spot beam that can light a path far ahead. In other words, they excel in "beam throw." This is important for fast-moving sports such as running or cycling. It can also make a difference in hiking on an unfamiliar or challenging path, where you want to see trail markers far ahead.
For task lighting -- such as checking a map, reading, cooking or changing a tire -- a wider beam is much better. Headlamps that excel in task lighting usually use an array of three to five 0.5-watt LEDs. You don't see as far ahead, but you get a wider area of light.
A few headlamps provide both types of beam, either by diffusing a spotlight for task lighting, or by using both one big LED and an array of smaller ones. This still involves some compromise, since a diffuser dims the light more, and using both types of LEDs increases the weight. Heavier headlamps often mount the battery pack at the back of the headband to balance the weight, which is fine for many purposes, but uncomfortable for some applications (such as reading while lying down or consulting a map in a car seat with a headrest).
As if this beam width and battery-pack location didn't complicate your choice enough, there's also the color of the light to consider. For most uses, white LED light is ideal for preserving good color discrimination. But for preserving night vision, reviews recommend headlamps with the option of red light. Some headlamps provide one or two red LEDs for this purpose, while others use a red filter, but many headlamps completely lack this option.