Just as an update, after reading this thread, I went ahead and ordered a Hubbub Mirror. I'd tried bar mounted and glasses mounted mirrors before and disliked both. So after reading the above review I decided to try this helmet mounted one. I wanted to wait until I'd used it for a good part of the season before giving it a review here.

My verdict: I really like this mirror.

I will say that when it first arrived I had a bit of trouble getting it locked onto my helmet properly, even after watching the company's youtube instruction video. And even after I solved that problem it took me a few rides and a lot of fiddling and bending to get it fine tuned just right. I had so much trouble getting it into position in the beginning that I almost gave up on it. But to be fair to the product I believe my trouble was almost certainly operator error. YMMV.

Now that I finally have it attached and adjusted properly I really like it. It gives me a good field of view to the rear (I can see the whole road behind me) and it doesn't vibrate much. So far it has stayed in position and hasn't needed any corrections.

A good mirror imho.