Being an all year round commuter, regardless of weather (although snow does stop me riding) in winter I tend to set off n the dark and head home in the dark. The Cateye Rapid 3 is a light I have seen on many bikes on my commute and lots of other Bromptons.
Things a fairly simple. There are three LEDs which give off a very bright and easily seen light. Three modes: rapid; flashing and constant keep things simple.
I like the way it can be mounted vertically or horizontally simply by turning it 90 degrees in the bracket.
I got it for £14.99 which I think is pretty good. It also comes with an AA battery and will last up to 20 hours in the rapid mode I will probably use all the time.