I find mine invaluable. I don't use it for major manoeuvres, such as right turns, but I do use if for constant awareness of what's behind. In busy traffic, it possibly adds little value as the answer to "Is there a car just behind me?" will always be "Yes". On rural roads, with not much traffic, it's extremely useful to know whether or not there's anything approaching from behind and it works perfectly for this. Again, it's the constant awareness thing. If you drive a car and choose not to look in the mirrors every few (single digit) seconds as a matter of course then don't bother with a mirror on a bike as it's this role which they fill.
For me, provided my mirror side leg is 'up' the view is very good, if narrow, with the Bike Eye. In practice that means that the view is fine when pedalling in the same way as you can see through rotating wheels, etc.