The lumen rating tells you how much usable light is created. While having more light is always better, how that available light illuminates the trail - commonly referred to as the "beam pattern" - is far more important than the raw power. A super powerful light that puts its light in the wrong place is essentially useless! For example, a light might provide a powerful long-range spotlight that offers no peripheral illumination; or the other way around. And if the manufacture is not showing you real life pictures of what you get with their light, be very VERY cautious.

A couple additional tidbits for the tech savvy:

1) A higher watt rating doesn't mean more light. Example >>: A 15 watt Halogen gets blown away by a 10 watt LED. Also, the higher the watt rating, the faster your battery drains. More about watts >>
2) Consider how the light rating is stated. Some will talk real-life trail projected lumens, while others state a lab value of just the LED given to them by the bulb manufacturer. What good is lab data when we ride down trails? In the real world optical losses and operating temperatures will lower this Lumen figure significantly. Lab numbers will always be much larger, and therefore better for marketing. More about lumen ratings >>