I do love my Take-A-Look mirror. Accidentally left the house without it yesterday and was annoyed by it's absence the rest of the day.
I also agree with the folks who place a higher priority on their mirror than on their helmet. The helmet's all about surviving or minimizing the damage in an accident. The mirror is about avoiding the accident. I know which I'd prefer. Not that all accidents are avoidable. Not that I know of any accident I avoided with a mirror. It's just my preference.
That said, it's what you're comfortable with. There's always someone who has this piece of gear or that practice that is absolutely essential, and anyone who doesn't follow suit is a fool. I try not to be that person. I don't know that a mirror makes me any safer. I know that I feel safer and that I'm more aware of other traffic as a result. But I can really only think of one time that I actually used my mirror to make the decision to bail off the road. Mostly it gets used for lane changes, which can also be done with a turn of the head.
So does it make me safer? I really don't know. It does make it easier to move through traffic, but it doesn't mean it'll do the same for everyone. Just like some driver's can't let the cell phone go unanswered in the car, maybe some cyclists can't help but ignore where they're going in favor of staring wistfully at the little bit of reflective stuff showing where they've been.
Does it affect my ability to look cool? Probably not more than any other choice I make about my wardrobe and choice of transportation. Cool isn't one of my goals. If it was, I'd probably lose the mirror, the gears, the helmet, 50 pounds, etc. But until then I'll keep the mirror because I like it.