There's a lot of different perspectives on it so here's a minority one. Most of my night riding is commuting, and most of that is not on the streets in traffic, so it's more important to see where I'm going than to be seen. I'm using an "Ultrafire" LED flashlight from Deal Extreme (ships from Shanghai). The light, batteries and charger were about $35 and it's nominally 500 lumen. In reality it's likely about half that and throws a non-adjustable narrower beam than would a headlight. That's fine for lighting up the road but not so good for motorist visibility.
Since my cycling philosophy is to not spend on it that's just about the best value I could find for adequate lighting.