My two Thugs use one of these cat flaps to get in and out of the house and we are all so glad ! It wasn't cheap mind (around �100) but since the girls don't have to wear heavy magnets (which in themselves are quite expensive to replace) and only they can get in, they are happy Thugs !
All you have to do to is get your cat microchipped, then post him/her through the cat flap, holding a button down on the flap and then the microchip number will be programmed into the cat flap. You can have 12 cats programmed at any one time, and can easily delete them if any are rehomed or pass away. The flap itself is virtually identical to an ordinary cat flap but there is a little *projection* on the outside that houses the techie bits. It was easy to fit and just needs to be plugged into a normal mains power socket - it comes with a 2.5m cable but you can buy a longer one if you need it.
You can programme the flap for a variety of different modes - like no going out at night (useful for bonfire night) or *vet* mode (no going out at all, cos you are going to the vet tomorrow hehe) etc.
Well worth every penny, in my opinion !