I used a Light and Motion Arc (HID) light for a while and liked it. Quit using it because, due to other riders filling the bike rack on the bus, I had to start locking mine up at the stop.
Recently I bought a Light and Motion Seca 700 for long-distance commuting. I wasn't sure about LED tech, but I'm now convinced it's viable. The beam spread is excellent, it starts instantly, and can be effectively dimmed. For trail use early in the morning, its 10% power setting is just fine. On the road, any time my conspicuity is questionable, I run it on full power so people see me.
I think it's too expensive, though. Were I to do this over again I'd go with DiNotte. I have their 400 taillight and it's excellent. They offer a headlight in the same package and two of those are cheaper than the Seca.