I've always thought the ones that clip on your spex are a good idea, but it wasn't until last summer that I finally got round to buying one. A few minutes up the road was enough to disabuse me of any idea that they're an aid to road safety.
The problem is that when you're hunched over the bike, the mirror is below shoulder level and not far enough to the right, so your view is blocked by your shoulder and ear. Attempts at tilting my head left me wobbling and swerving all over the road. It also creates a huge blind spot obscuring your view of traffic approaching from the right at a give way line.