I have one of these on my Jamis and I like it. It blends in without looking like an appendage. The field of view is not large, but it is enough to let me know something is coming, as long as I remember to glance down periodically. It is just a ball in a rubber socket, so it needs readjusted after I hit a large bump.

I also have a Cat Eye bar end mirror. It is large and, since it is an actual glass mirror, has a very clear view. It is also very stable. It may not work so well to see past your legs if you have narrow bars.

I have a couple others that are mounted on arms away from the bars, but they vibrate more than I like.