I just one too. I got a green for the front wheel. I figured my panniers would block the back one too much that it wouldn't be worth it.

They are nice because you can easily take them on and off, so I at work I can just throw it in my bag with my other lights and not worry about it being there when I come out.

I got the green because some communities frown on blue as it is for fire and police, so I didn't want to take a chance and I read a review that the green was much more visible.

My bike has a regular reflector on the wheel as required by NYS law, so I just go one to balance out the wheel.