We had an intruder cat that came into our house after our own cat, it not only attacked our cat, it also did its business both number 1 and 2 in our house.
I got our cat chipped for £16 with the vet and fitted the cat flap and its done the trick, I did not have any difficulty fitting it, the took a day for our cat to get the hang of using it.
I was woken up in the first week by cats making a noise and found our cat the right side of the door and the intruder cat not able to get in. RESULT!!! In short I am very pleased with the product, so I gave it a 5 star rating.