I never saw the point of mirrors until I bought my first recumbent in 1990. The mirror became second nature, although as above, not a replacement for a proper look) I soon found out that I missed them on my other bikes, and fitted them. Now all my bikes have mirrors (usually Mirrycles) fitted.
However when I bought the Nomad last year, the bar end grips precluded this and I bought a Bike-Eye. It has survived two long tours, a crowded garage, and several train journeys without any damage.
As for view - that is the drawback...
You need to "cock your leg" to get the best benefit, as otherwise you only see through the narrow tunnel of your thighs. Apart from that with head movement, the angle of view is reasonable. Luggage is a problem as the top of the panniers exclude part of the mirror, the remaining view is adequate for most things though. A low fixing rack like the Tubus would solve this.
All in all, if you can fit a Mirrycle, but if you can't then the Bike-Eye is a suitablecompromise.