I've used about every variation of mirror over the years, but one thing remains constant -- I feel vulnerable without one. One benefit that isn't obvious 'til you've used one is that you'll know when there are no cars are coming, and the road is yours.
My favorite is the Take-a-Look that others have mentioned. Superior quality and since it mounts on on your glasses, you can scan the road behind you and there's almost never any vibration to blur the image. I've even used it in road races to keep an eye out for wheel suckers.
Whichever mirror you choose, once you've used it for awhile, you won't want to ride without it.