NiteRider

It's better than I'd dared hope for my particular needs. I don't know anything about the seatpost attachment components, as the option I needed/use is the "belt clip" style attachment that's part of the light itself. I clip it to a webbing loop on the back/rear of my under-seat bag, and from that location, it gives an excellent view to traffic. I don't routinely ride after dark, but wanted this light for increased safety for daylight riding and into dusk on occasion. Especially for the modest price and light weight, I'm really impressed with how bright this tail-light is, and the several different types of "flashing" light options in addition to solid "on".

It's a back flasher light. Not a high-tech thing (i.e. like Di2 or carbon wheels). But it works. Easy to mount on my bikes - I have multiple bikes (Cervelo S5 - aero seat post and a Caledonia (standard round 27.2 seat post). Battery lasts a long time. Sometimes I forget to charge it and I am amazed that it lasted for that entire ride. It's really bright. In the morning when it's dark, I turn on the NiteRider Sentry Aero 260 Rear Light, the light hurts my eyes because it's so bright.

I would like to give this light 5 stars like everyone else, but the mount strap is too short. I was able to stretch the strap and get it to hook into the first hole; but it is too tight and I expect it to break sooner rather than later.

Also, the the base of the strap which attaches the strap to the light is curved on the inside, and the back of Specialized seatpost is flat.

I cannot recommend this light for Specialized road bikes.

I chose this light because it has a hook mount, which I use to attach it to my helmet strap. The hook is very secure, it's never come loose once in over a year of riding. Even though it's small and light, it puts out a lot of light. I just bought a 2nd for my commute bike, where it mounts on the rack. I've had it on for many hours during long rides, it has never stopped working.