They look cheap, come loose, rattle, add a touch of weight and drag. And they don't work.
If you're going to ride at night you need much more than reflectors.
I rarely ride at night, but if I do, its with a front light, rear light, and wearing illuminite clothes that light up like a christmas tree. Way more effective than the cheap reflectors tossed on by bike manufacturers to comply with CPSC regs.