Since the more expensive one got chewed up, I bought 3 of these and keep them in handy places (like on my key chain.) It is pretty easy to clip one onto my dogs collar on a whim. This way we can tell where she is and get great amusement out of watching the bouncing red light. Battery life is pretty good for occasional usage.
This looks pretty sharp, and it was easy to see my black dog at night from pretty good distances. It just didn't last. I noticed that the black nylon over the end of the light area started fraying badly within a few days. The blue part quickly deteriorated as well due to general dog play. The battery pack sleeve was not secure enough, and the battery pack disappeared within a week. Maybe a velcro wrap around it would help. Too bad, because I really wanted it to work. I guess I will go back to the little red clip on.
I had it tightened into its shortest length, where it was about right for my 48 lb dog. Buckles and webbing were a little wide for her. Call it large - not medium, or at least list dimensions. Also the weight of the tags rotated it, so the light was under the neck. It would be better if the tag ring was near the buckles so the light would be more on top.
I would recommend this for more sedentary dogs with the caveat that the battery pack should be secured a bit more.