I bought these so that I had some compact lights that were bright enough for city use, but that you could easily remover and fit in your pocket so they don't get stolen. The lights are very small (slightly wider than a 2 pence piece) and also are very bright considering their size (sufficiently so for cycling around a lit city so that you are visible).
However after a few months use I am left with only the front one still working. This is because of the way the elastic is attached onto the light. The elastic is attached to a small key ring which fits onto the back of the light through a small screw. The overall build quality of the plastic is quite poor, the seal between the two halves doesn't match up very well and the plastic has crumbled around the screw threads. This means the metal key ring will no longer stay on the screw at the back of one of mine. Overall the screw heads and plastic casing are not of high enough quality.
The other problem I had which I think is possibly another design flaw, is with the on/off button. The button is very flimsy and needs to be pressed quite carefully, one of mine stopped working and had to be pressed down quite hard with the end of a pen to get it to turn on. It has subsequently become very difficult to turn on and off.
I ended up deciding to keep the front one in use until it stops working too, and for the back light I replaced it with a similar but better design by Electron Backupz (Electron Backupz Seatpost Mounted 6 Led Light). This is a considerably higher quality product, with a more robust feel, higher quality button, and quality screws. It also mounts horizontally to the seat post which is better than vertically, and has 6 leds giving an even brighter light. It was marginally more expensive but worth it for the build quality.
Was torn as to how many stars to give this, as it would have been a great product were it not for the two design flaws that seem to quickly render it useless.