Love it. I think I'll probably always ride with a mirror. I like getting a preview of what's coming from behind. It's especially nice when I have a lot of stuff in my backpack and it is kind of hard to actually look behind myself. It's also nice when I have to stop at traffic lights, and there are punks on the corner acting like they might give me a hard time. I can keep an eye on them whilst appearing to ignore them and refusing to engage with their bullsh*t.
It makes it so much easier to plan what to do in traffic, and it's nice in no-drop group rides, I can keep an eye on things if it looks like we are starting to lose people, I holler. I'm also usually the first one to call out "car back."