I had mirrors (Sprintech bar ends) for a month or two. I found them helpful on the road to see incoming traffic and also to see what's going on in the pack behind me when riding with a group. They were also helpful while racing for the same reason. I wouldn't say it was a 'night and day' difference or anything of the sort, but in some situations they helped and I can't think of a single circumstance in which they were detrimental.
Then I had to take the plugs out to change the bartape and the plugs broke, so I'm back to being mirrorless. No big deal either way.