Addendum: Ihave seen people advocate high power lights (eg: magicshine at 900lm) for "serious trail running." Well, people use lights like that for mountain biking and I really doubt you're going to approach those speeds on foot (could I be wrong?) If you're a fast runner and actually need the lamp to navigate and not just to avoid obsticles than maybe something like the EOS or even brighter would make sense, but I'm vary curious how the EOS does as I find the flood excellent for picking out not just my path but also each foot placement on rough terrain.
I'm the one who suggested the Magicshine (though it's really 550-600 lumens). I think what's more important than speed of travel is how much you want or need to see into the darkness beyond what you have to react to in the next couple of seconds. I know there are many night orienteers who read this forum (including me - though my club only hosts one night event a year), and for that you need to see terrain and objects quite a ways out. (A forum member has linked to a 3000 lumen homemade light for night-O.)
Years ago, I used to do long trail runs at night without any lights at all, but I had to psych myself up to do them. Then I got the Princeton Tec Solo and, a few years later, the Aurora. Those headlamps sometimes enabled me (barely) to see the ground in front of me better, but the overall experience was similar to no light at all. I usually ended up turning them off and was better off for it.
My P4 modified Eos was substantially better and made running on trails comfortable much of the time if the trails were familiar ones. Frequently though, I find myself in condtions that absorb the light of the modified Eos, to the point that it seemed like it's barely on at all - kind of like going back to the Aurora. I'm sure most of us have experienced darkness like that (imagine wet asphalt, dark colored trees, dark dirt...).
The Magicshine (or lights in that brightness range that orienteers, the most intensive night runners, use) just makes night running a much more relaxed affair. I can spot moose and bears or other critters from way out instead of almost running into them with the Eos. I can match what I see on my map with what is in front of me if I'm orienteering. It's just a lot less confusing and a lot more comfortable, especially on unfamiliar terrain.
Even with the Magicshine, I experienced real light absorbing conditions on a ski in December that brought the range down to just enough for slow skiing (not faster than running pace). It was far into twilight, but not quite night and snow was being blown by very high winds (50 mph?). On the plus side, I wasn't getting flashback off the blowing snow because the light is high enough above my eyes. Without the Magicshine on, I couldn't see the features of the wind carved drifts that I was skiing on at all.
By the way, on many trickier trails, I sometimes get a kick out of keeping up with mountain bikers. If there are decent climbs and no long downhills, I can get pretty big gaps on bikers on the uphills that I can maintain on rolling terrain. (I used to be kind of fast - 6 miles in 29:28.)