Excellent low cost item that increases side-on visibility in darkness and low light.
I have some great front and rear lights on my bike but was always concerned on roundabouts at night as car drivers are so involved in their sat-nav, ipod, central heated, 7 airbag safety shell that they barely give a glance on roundabouts and if a 100w halogen isn't burning their retinas they just pull out. These green little fire-flies of light create a great illusion of a circle of light when travelling at any decent speed and make you much more obvious to other traffic. The green also helps because you rarely see any green lights at night and it makes car drives think a little (never a bad thing).
Battery life is OK but easy to replace and you can buy a dozen on Amazon for a couple of £s.