I have a mirror mounted into the end of the bullhorn bar I have on my bike. I find it's very useful to keep track of what's coming up behind me, be it a car that might want to blindly turn right across the bike lane (a common problem in Shanghai) or, as the bike lanes in Shanghai are shared by bikes, electric bikes, and gas scooters, or it might be a fast moving scooter that I need to move over for. Given how chaotic the streets in Shanghai can be, the extra situational awareness that a mirror adds for me cannot be overstated.