The Planet Bike SuperFlash Micro USB Tail Light is a tiny red light that is quite bright. It comes with hardware that allows it to be mounted to a bike seatpost or seatstay and a clip for mounting it to a bag or clothing. I used several zipties in combination with the clothing clip to attach the light to the back of my helmet.
The light's packaging makes wildly optimistic claims about runtime. Like pretty much every LED "blinky" on the planet, this light can be set run in either flash or steady modes. Some folks find flashing lights very annoying and various randonneuring events and some European countries require lights be run in solid modes. I tend to favor the steady mode of light myself, so I figured I'd test the runtime to see how long a charge really lasts with the light set to solid. When blinking, the light is effectively off about half the time so naturally the light burns longer since the battery gets to rest between each light pulse.
The package claims a 2 to 4 hour charge should give 36 hours of runtime. Experience has taught me that such claims are inflated and I figured they were basing this on flashing times, so I ran a couple of tests on the light. The charge times were accurate.
I plugged in the Mini USB cable and charged the light. I ran the test a couple of times, charging the light fully until the blue LED stopped flashing and burned solid. I then let the light run in solid mode up until the battery depleted to the point it switched to the flashing low battery warning flash mode. In both my trials, the results were the same.
The charge time was about 2.5 hours. The solid burn time was nowhere near 36 hours. After 7.5 hours, the light switched to the flashing mode. 55 minutes later, the light would go out. Obviously, this is far short of the claimed runtime.
On the plus side, the light is well made and seems nicely weatherproof. It retained full brightness for the 7.5 hours and the low battery warning flash is still plenty bright to get most commuting cyclists home on a dark night.
Since I never believed the 36 hour claim, I'm not too disappointed in the short runtime. For my purposes, as a helmet light, I appreciate the light weight and with my current, brief commute I won't have to charge the light all that frequently. But randonneurs or others looking for a light that can reliably shine steadily through long dark nights should look for something larger with more proven battery life.