I've had them on bikes for 20 odd years now, serious car crash hurt my neck, Ihave spent loads on different types, new ones to market etc - trying to be sure I have the best and most effective one(s) I can find.
Raleighnut is right to get the popcorn out, Mirrors cause nearly as much controversy on forums as helmets, so just to start it off
The negativity mirrors attract seems to come mainly from people who have very limited use to comment from (I tried it for a couple of weeks and....) or picked a cr@p one and have generalised from that about the whole range of different types.
The other tired old canard that gets trotted out is 'well I turn my head and it works fine' as if having a mirror and turning ones head are mutually exclusive or everyone is blessed with a fine healthy Owl like range of movements in their neck.
You will find that fitting a mirror to your bike does not lock your neck into a front facing rictus like grip that it is impossible to move from and you can still do the lifesaver looks and use your head and eyes as a means of communicating with the driving population around you.
here endeth the rant.
The best bike mounted mirror for the job varies, dependent on your handlebar type but in my experience will be one that plugs into or grips directly onto the bar without an arm of even the shortest length which will introduce vibration and have greater chance of being knocked askew.
You also need to consider the (often) ball joint between the bar plug attachment bit and the mirror piece, some are very flimsy and simply not up to the job of holding the mirror firmly enough in position to be of any use
This is a perfect case in point for that: lovely size mirror, direct plug in, omnidirectional but, from direct experience, the ball and clasp piece is disappointingly weak and lets down an otherwise good bit of kit
I would strongly suggest you consider the Zefal range of mirrors:
The Dooback for flat or cowhorn bars - it has a ratchet mechanism to hold it out, which is a great thing in use in whatever weather but is a tad restricting to use on properly raked/moustache style bars.
The
Spin and
Spy are a smaller mirror surface (of the 2 I favour the spy type) but both more versatile for non straight bars, the spy with its very effective strap grip is the best option for butterfly bars if you have them.
*Edit - links are for indication of what the mirrors look like only, not an endorsement of the vendor or the best price etc - hunt around for the best deals *end edit*
For drop bars (without bar end shifters) the
Sprintech bar end mirror is designed to have the optimal mirror shape and size and is very good and solid when riding although the ball and socket joint is susceptible to jigging if you lean your bike up against a cafe wall or in a crowd of lean to parked bikes.
The Spin will plug in to the bar end too and is a bit less susceptible to the jiggling vs a less optimised drop bar mirror shape. A Spy will fit the drops or around the hoods without problem and can be used with bar end shifters in place.
I've never found a Cateye or Blackburn (rubbish IMO) mirror that I'd give handlebar space to ahead of any of the Zefals or the Sprintech.
The cheap silver motorbike type on an arm ones are akin to putting your money down a drain.
There is an old thread on here talking about the BikeEye frame mount type too, I took one off a lad on here who tried and ditched it, i did too very quickly - needing to interrupt your pedal rhythm flapping a leg out, it restricts what rear luggage you can use and its central position is just not an intuitive place for your eyes to flick to (albeit Chris Froome with his fascination for his bikes top tube might do ok with one
) I would suggest that they are a waste of more money than the motorbike flappy arm ones.