i just put a handlebar mirror onto my get about town bike and took it out for a test ride yesterday evening. here are my first impressions:
i can't just glimpse at the mirror briefly
it isn't always pointing the right direction so i have to change direction/turn handlebar to point it
i swerve when i look at it (turning the bar or just not looking ahead)
it only tells me there are cars in the lane way back which is useless info
cars right up close that affect the riding are close enough to hear and don't always show up in the mirror
generally it is very distracting and takes my focus away from looking where i should be looking.