This is a great, great, great light. It is bright, and casts light in a pretty wide arc, so it is visible from most directions. The flashing mode strikes a good balance between visibility and battery life, which is nice because CR2032 batteries are relatively expensive and this takes two of them. I've previously used Nite Ize SpotLit (great visibility, horrible battery life, died quickly), PupLight (good visibility, good battery life and ran on AAs, but my dog managed to lose it in the woods somehow*), and Cateye LD150 Taillight (horrible visibility (it is unidirectional because it is a bike light), good battery life and ran on AAs; I was getting desperate).
As others have said, this needs to slip on over the collar strap. Which is kind of annoying if you don't normally use a "belt" style collar. What I did was buy a Rogz Reflective Harness and then use some nylon webbing and rivets to attach the light. It was quick and easy.
3 month update: Over the course of a month, I bought five of these (three dogs), and they are all still working great. Haven't had to change the batteries yet, despite pretty heavy use. My one dog has taken his swimming a few times, with no apparent problems. Still super happy with these.