As a bike tourer and everyday bike commuter, I use mirrors. I almost can't get on a bike without one. I've gone through many over the last decade since I got back into bikes, and my hope is that this synopsis will help someone short-circuit the process. My perspective is informed by the fact that I wear glasses, don't wear sunglasses, and all of my bikes now have drop bars.
Before I get too deep into this, note that mirrors do not need to do much to be useful. A single bit of information, whether a car is back there or not, provides the majority of what you need for safety. Second-order bits would be distance, speed, etc. The first bit is absolutely crucial, so if the mirror bounces too much to read, or the field of view (FOV) is too small to capture much of the road, the mirror is at best useless. It can be a negative if you think there's no car, but there really is. Large FOVs require either the mirror is mounted close to your face (helmet or glasses), is enormous (nobody does this), or is convex. Convex mirrors give problems w/ determining distance, but are perfectly good for determining whether something is there.
Here's my take:
helmet mounted (third eye etc), attaches to helmet w/ sticky pad
Pros: no vibration, good field of view, easy to point.
Cons: cheap plastic attaches to helmet, easy to bump, break, have to adjust every time I get on the bike, difficult to get a good position, have to be careful w/ the helmet.
glasses mounted (take-a-look is the best)
Pros: the gold star for function, great field of view, no vib, light. If you have a separate pair of sunglasses in your handlebar bag, and wear them every time you get on the bike, look no further.
Cons: Since I don't have a separate pair of glasses just for biking, I need to take it on and off every time I get on or off the bike. For those of you who care about looks, they are seriously goofy-looking. Rubber sheaths on the "clamp" fall off and get lost.
handlebar mounted
I'm left w/ this category, so here is my take on several that I've tried.
mountain-bike barends: bar ends are way too far back to be of use
sprintech barends: Very cool looking, but I find them too close to my body (have to look straight down to use them), they are small, and I bump them w/ my knees.
STI-mounted (mirrcle): right position, but I've had bad luck w/ them breaking, and I don't currently use STI.
BlackBurn Road Mirror:
pros: gets great reviews, easy to mount/unmount, usable
cons: Vibrates too much, doesn't stay in position on big bumps, sticks way out (hard to get through doors), and mode of attachment, while functional, flat (so limited FOV), is cheap and ugly.
German Ultralight Mirror:
.
Frustrated w/ the blackburn, I went back through the amazon reviews. Among all the glowing ones was one guy who said it couldn't hold a candle to this mirror, so I tracked one down and tried it.
Pros: doesn't stick far out (vertical orientation), little to no vibration, solid attachment, convex (decent FOV)
Cons: convex (takes practice to estimate distance), cheap plastic, FOV still nowhere near take-a-look.
I have several different mirrors, but only one german ultralight, which I mounted on my heavy commuter bike. I liked it, but wasn't blown away. Without realizing it, however, I've avoided using any other bike over the last two weeks, even for 20+ mile rides, because I'd have to use a different mirror.
Basically, it does everything pretty well. Convex+no-vibration means that I get a good sense of what's behind me and it's clear enough that I can estimate distance well. The mirror stays put with road bumps, and mostly even when bumped directly. Eyeglass-mounted mirrors are easily angled around so you can look left or right by turning your head. A mirror attached to the handlebars is less maneuverable, but the convexity mean you don't have to maneuver it very far to see left or right. I don't know how durable it will be, but I've read good things about this in other reviews.
Bottom line is that I ordered another for my touring bike.