I commute year-around and in the winter it is completely dark on the way home. I like to have two independent tail lights in case one goes out along the way. It is lightweight, easily attaches securely to the seat post and has been trouble free half way through the winter. I'm not sure one of these is bright enough for night riding on roads with speed limits exceeding 30 MPH hence the four star rating instead of five.
Wildly simple to attach, move and use. Charged it once two weeks ago, haven't had to charge it yet. Simple operation (I often forget to start it before the ride, easy to simply reach down and double-tap the lens to get it going). Only improvement I could ask for is a slower flash mode - the flash mode lasts forever, but it's a little frenetic for anyone following. Slower-paced flash would be great. Still a great product.
Can't ask for more in a taillight. I can use it for two hours a day for at least 2 weeks without having to charge. This is perfect. Super easy to swap between bikes too. I put it on my wishbone seat stay or bridging two normal seat stays.