There are two points of view on this and the best way to decide is to experiment with a mirror and see how you like it. Some cyclists are opposed to mirrors because they believe that they offer only a limited view of what's behind, and one that isn't as reliable as what you'd see if you turned around and actually looked.
There are other cyclists however, who swear by mirrors and wouldn't ride without one. They'll tell you that with a mirror, you can instantly see if the road's clear behind and know when it's safe to take the whole lane (which is most of the time in lots of places). Also, you'll be able to tell what vehicles are overtaking you without having to turn around and look (always a tricky maneuver when some loud truck is approaching because you don't want to swerve). And, always knowing what's going on back there can help you ride more safely. For example, if it's a narrow road and that large truck seems bent on passing, you might duck in a driveway to let him by.
A mirror is also handy when you're riding with family or friends because you can keep track of anyone who's falling behind. Or, you'll know if you're the one bringing up the rear.
There are several types of mirrors available from types that mount to the handlebars, to those that attach to the helmet or to your glasses. We've got a good selection to choose from and we're happy to show you how they work.