Around the beginning of 2009, I got a new light to go with my Nexus generator hub. It's the Busch & Müller Lumotec IQ Cyo, an high-power LED light with a carefully engineered beam pattern, and it's by far the best generator light I've used or seen. There are a few others that are supposed to be about as good, maybe even a bit better, but at about twice the price (~$200 vs. ~$100 for the B&M IQ Cyo, so it seems like a very good choice in comparison.
Here's a detailed review, including a few issues specific to icebiking.
The main thing I like about this light is that it's really bright. It does that by having a very efficient modern LED, an efficient electronic switching power converter to match the generator output to the LED characteristics, and a high-performance reflector that directs every photon that comes out of the LED to a useful place on the road. Previous generator lights I've used provide enough light in most conditions, but not enough for difficult situations. The IQ Cyo provides more than plenty in most situations, and more than enough in difficult situations.
Some comparisons I can make directly: -Vastly more light than a traditional halogen generator light. -Lots more useful light than the old Solidlights 1203D, an LED generator light with smart electronics, but with a much wider beam and lower efficiency LEDs. (Solidlights have just announced a new generator-powered model, the XB2, which has a narrower beam and higher efficiency LEDs, but, at $237, it is twice as expensive than the IQ Cyo.) -From subjective memory, not a side-by-side comparison, a little more light than a tight-beam-pattern 6 W halogen DIY battery light I made a while back, using optics from a good generator light. -A little more useful light than typical 10~15 W halogen battery lights. -Significantly less light than a Light and Motion ARC HID lamp, but not all that different as far as useful light on the road, given that the ARC spews light everywhere.
The beam pattern is carefully designed to uniformly iluminate the road in front of you: rectangular, brighter towards the top (because that is illuminating the road further away), and then ending abruptly so it doesn't blind oncoming drivers. It works really well in most situations. Although I have it mounted to my fork crown so that it turns with the steering, it doesn't turn quite enough for me to be able to look ahead well when I'm turning a sharp corner. It also isn't great for reading road signs, or for avoiding branches if you are riding off road. But for most road riding it's just right, and gives ample illumination of the road.
There are actually two choices of beam pattern--one directs a little more light downward immediatlely in front of the bike (3 to 10 feet away) whereas the other aims all of the light further ahead (10 feet and beyond) I opted for the one that directs some light down nearby, thinking that in snowy conditions I'm looking at the nearby surface characteristics as much as I'm looking at obstacles further ahead. I probably made the wrong choice there. With the light mounted on the fork crown and with a 37 mm wide tire with a fender to match, mostly what I see on the ground near me is a big shadow of the wheel. But I might move the mounting point...
The light worked 100% reliably through cold, snow, salty slush, rain, etc. The only problem was that, mounted on the fork crown, it got sprayed when I rode through loose snow or slush. The lens got fouled--covered by slush which then left behind a salt crust. The optics didn't work so well with muck over the lens. Fortunately with a water rinse it's pretty much as good as new. But I'm looking for a better mounting point, nearer the handlebars. Hopefully that will make the nearfield lighting option more useful as well, as the wheel won't cast such a big shadow.
There are models without switches for use with a bottle generator that you physically disconnect when you aren't running it, models with switches, and models that automatically turn of during daylight. I opted for the manual switch, because I find the dynamo drag is so small that I often leave it on for extra safety during the day.
It has a very strong "standlight" feature--it has an ultracapacitor inside that stores enough energy to keep the light on for a while for when you are stopped at a light or the like. It's much brighter in that mode than the old solidlights unit was, and it can run for longer (haven't timed it but they say 3 minutes). It could serve as an emergency light for fixing a flat or the like, if you are quick at that, although most mounting schemes have it rather permenantly attached to the bike. The only thing I don't like about the standlight is that I feel I need to turn it off when I lock my bike in a public place, so that people don't see it on and worry about my battery for me.
I bought it from Peter White. Scroll down two items from where this link takes you on the page. It has nice diagrams of the beam pattern options. http://www.peterwhitecycles.com/schmidt-headlights.asp#fly
I found this page with pictures of one taken apart very interesting. http://fotos.mtb-news.de/sets/view/12234/page:1
The ~$200 competitors--Supernova E3, Schmidt Edelux, and Solidlights XB2--are probably fantastic too. But this is little more than half the price and likely about as good.
LEDs are likely to continue to improve rapidly, but the IQ Cyo is good enough and reasonably priced enough that it seems like a good time buy one and not need to think about getting another light, replacing bulbs, or replacing or charging batteries for a very long time.