I use it in the day time too - it's really bright. Fits the triangular carbon seat post on my Cervelo SLC.
(Sadly, I lost mine today on a rough chip sealed road. Guess it didn't click into place securely enough after changing the batteries... Time for a new one!)
I clip this light to the back of my rear wheel carry box. It works great, is bright and with a selection of settings for either blinking pattern or solid light, I know I am seen by vehicles coming up from behind.
I like the sturdy clip and the sealed outer shell of the light. I like how it can essentially go anywhere on my bike.
I am not a fan of plastic, but in this case, it being so hardy it does what I want to do and is very strong....
I like this light because it attaches and unattaches easily without tools. It has 3 blinker patterns (not sure why more are needed?) and puts out plenty of light. I ride in the city and feel more secure at night as a result. I recommend it, especially for the price.