I haven't found it useful commuting in traffic. By contrast, I use mirrors heavily when I drive. That's due to early training so I guess I'm not yet adept at looking at a mirror quickly in fast-paced circumstances. I find the mirror most effective when I'm on a shared path to know if another rider is coming up behind me, and when I'm riding with others to keep check on how far apart we are.

For those keeping score, I'm using a Bike Peddler eyeglass mount mirror. My wife used an STI-mounted mirror until a fall broke the mounting lug off the top of the brifter. She tried a helmet-mount but it wasn't working for her. She's mirrorless for now.