I don't think I'd really want to go with less light than my MagicShine (probably 400-ish lumens, as noted by other il-lumen-aries above ).

100 lumens might actually be enough for riding in complete darkness, as long as you're not doing any mountain descents or MTB tricks. Where I find I need more light than that is in heavy automotive traffic or in areas with irregular illumination of the ground. That means there's more than enough light to kill my night vision, but I still can't necessarily see that upcoming pothole or ice patch without a good light source of my own.

Looking at the comparable numbers for cars--quick check of Wikipedia--Halogen automotive headlamps put out about 1500-2000 lumens, and HID systems are around 3000. All of these numbers are per bulb, so most bike headlights aren't anywhere near car-headlamp territory (though some of the brighter ones can be a bit of an annoyance to oncoming MUP users if they are aimed too high).