I should have bought one of these months ago when the problems with the large black tom cat started and one of my son's friends suggested the microchip cat flap! It's brilliant. So easy to install, after reading the instructions thinking it would be complicated it took me approximately 3 mins. I was also impressed how easy it was to programme. My young female cat, although fairly feisty, was being hassled by the tom cat who regularly came in and stole her food, and even had the nerve to hiss at me late one night when I took him by surprise in MY house! We've had no problems since. She comes and goes as she pleases again, she quickly got the hang of waiting for the click as she approached the cat flap, which released the lock and let her in/out. If the tom cat tried to gain entry since it was installed, I'm unaware of it, he hasn't been in and my cat and I are a lot more relaxed. I was away for a week (son coming in every day to feed the cat), and did not have to worry about her peace of mind while I wasn't there. I could not recommend this highly enough and have told many people about it.
Followup: The cat flap has now been in use for 21 months and I've not had one single problem with it, nor any intruders. When the red light flashes I change the batteries, they seem to last for months but I keep a spare set (4 x AA) just in case. I'm moving house and have bought another of these cat flaps to put in there. The only downside is that it will have to go in an outside wall (current one is in the back door), so I'll have to fork out for probably 5 extenders, but it will be worth it in the long run - if my cat's happy, I'm happy.