I have one and am very satisfied with it, because before we discovered this marvellous invention, we were troubled by entire tom cats coming in and spraying the passage or the wash house!
The flap only work, of course if your cats are chipped, and if you have suceeded in getting each cat to go through the flap when it was set up to record their chip. Not the easiest of tasks. I suggest you rope in at least one other person who is used to cats to help with this part of the process, which fortunately only needs to be done once for each cat.
You can close the flap entirely, so the cat or cats can neither go in or out (useful when a visit to the vet is looming, as running round the garden looking for the potential patient is no fun), or you can turn it into a one way only door, which is useful when as I often do (read every evening) have one cat in, and one out, and want both IN for the night, or if you don't want the cats in while you are painting a floor, or working with a substance harmful to them.
And this marvellous invention has a warning light that comes on when the batteries are getting low and should be changed.