I got one of the cheap Halfords mirrors (
here) just to see if I got on with it. For the price, it's excellent. As a car driver/motorcyclist, I am used to keeping a constant picture of what is behind me at all times, and I missed that on the ebike. Turning right round for a good look isn't possible every few seconds, but with a mirror it's a breeze. The drawback with the Halfords item is that the stem is short, so it's easy to knock with your hand, and the ball joint isn't very strong, so needs frequent readjustment. I had to cut a hole in the end of the grip, but it's invisible now the mirror is there. But the picture in the mirror is clear and useful and I am glad I have got it. Now it has proved its worth, I intend to upgrade to a better one, and this thread has made me think I'll go for a Mirrycle.
One thing - the OP mentioned using a mirror to check before moving out round a parked car. I would say use the mirror to get a general idea of what is behind you, but before any manoeuvre that takes you out of your lane you should
always do a shoulder check before committing. They don't call it a 'life-saver' for nothing.