I started out with a helmet mounted mirror. I now use only handlebar mounted mirrors. It is far from obvious to me that the helmet/eyeglass mounted mirrors are superior.

The mirror area on the head-area mounted mirrors is fairly small. You compensate by moving your head to increase the coverage area. I dislike this because you are focusing your attention away from the road in front of you and I find it very difficult to pick up the area you are interested in with a quick glance. For me, it took a lot of concentration to use it effectively. It also puts a small plastic object in the vicinity of your eye, which is less than ideal in the event of a crash.

If you have a flat bar, the Mountain Mirrycle Mirror provides a huge and adjustable viewing area which is almost always pointing at the area that you want to see. It's easy to pick it up with a quick glance and it provides a stable and clear view.

I haven't been able to convince myself to try the STI Mirrycle because I don't like the idea of having something jammed in my shifter. It sounds like a recipe for breaking an expensive part, even if it doesn't degrade normal functionality. So, on my bikes with drop bars, I use the SprintTech drop bar mirror (having compared it side by side with the CycleAware roadie and concluded that the SprintTech gives a better image). The drop bar mirror, which mounts on the end of the handlebar, is much smaller than the Mirrycle products. It doesn't give you nearly as visible a rear image. It's easily blocked when you're riding in the drops and I often bump it away from where I want it to be pointed. It gives, at best, a hint as to what's behind you. However, a hint is generally all I really need. If I'm about to make a potentially life-threatening move like turning across a traffic lane, I turn and look directly behind me, regardless of what mirror I have.