Currently I use the Nite Rider Trail Rat. It's mounted on the helmet (can be on handlebars also) and is a really bright light, nothing compared to the HIDs though. I use it for night trail riding and also night road riding. The battery is smaller and is stored in my hydration pack, I could fit in a jersey pocket though. It has I think 2-2.5 hours of life. Overall, I'm really satisfied with the light, but it is around $150 I think (I've had it awhile, can't remember exact cost).

I have been on road rides where I've passed people with commuter lights. Honestly, I could hardly see the people. I'm sure it gave the rider enough light, but definitely not enough for other people to see them. If you are just looking for a light to get you through the last little bit of the ride when the sun is setting, and not full blown night riding, it's probably OK. If you're looking to do serious night riding though, i.e. a good hour or two, on the road or trail, in the dark, I would go for a higher performance light. Also, reflector are much easier for cars to see than lights. Get some reflective tape and put it on various places if you plan on doing night road riding, in the road.

I've been on rides where people have HIDs and they make me jealous. HIDs are soooo bright! If I could afford one, I would totally get a HID.