For the last 4 or 5 years I've used a Take-A-Look, but attached to my helmet rather than my glasses. I don't like to ride without it.

I don't use it to avoid getting hit from behind, but rather to assist in changing lane position. I like the way an MSF instructor described mirror use. He said "Mirrors have a one word vocabulary. They can only say No". The idea is to check the mirror, and if you see traffic, don't move. But if it looks clear, you still have to do a visual check. The reason is that all mirrors have blind spots, and the only way to be sure it's safe to change lane position is to do an actual visual scan.

Personally, I don't have a problem in maintaining control while taking a quick scan over either shoulder. Scanning is one of the basic handling skills taught in bike safety classes for kids as well as adults.

I agree with Chris516 that a mirror can be a distraction. When I first started using a mirror, I'd get so focused on it that I wasn't paying enough attention to things happening in front of me. It took a while to get used to using it with short glances.