After a three years or so, my trusty LED collars packed up and I decided to try these for off-lead night walks. After a couple of weeks' use in the wet, fog and cold I'm hugely impressed. I bought three of them, and the different colours work great for identifying my different hounds.
However, and this is the bit I wanted to share with you, choose your colour wisely - the blue and red I have are brilliant (plus my kids think it makes the dogs look like a police force), but the white one not so good. Reason? It's so bright it affects your night vision. It's really bright (which is of course a good thing!) and if your dog is close by, I find the impact of the white one makes seeing the ground more difficult and is also vaguely annoying in a way the red and blue ones aren't. In a sense, it's not a criticism of the product, I just wanted to share my experience, as if I had known, I wouldn't have chosen white.
I hope this helps, and happy walking.