We have a Staywell magnetic catflap. Our cat sometimes has difficulty getting in as he hasn't quite got the hang of making sure his head goes in first (as opposed to feet). Unfortunately our neighbour's cat has no problem - he just bangs straight through with no magnet and even with the locks on. He did it with such force one night that I got up the following day to find the actual flap on the utility room floor - he'd knocked it completely off its hinges. Managed to get it back on though. We now keep a log in the outside bit overnight (with the locks on - ours stay in at night) to stop the bugger getting in.
Replacement magnets are quite pricey - around £7 I think - so if you have a cat that regularly loses it's collar like ours it can get pricey. That cat now has no magnet and sits at the window to be let in!
I had one but my cats kept losing their collars (and the magnets) and it cost too much to keep replacing them. Also I found it wasn't that great and sometimes it would take them a few attempts to get in. I can't remember what make it was, sorry.
We had one, it worked fine : until the day I came home early from work and found the flea-ridden neighbourhood bike moggy in my kitchen eating Gillespie's (our cat) lunch - sporting the exact same staywell magnet.
Take a walk, check out the neighbours doors, and buy a different brand
We had a succesion of Staywell ones (straightforward magnetic ones and then one where only specific, programmed magnets work) but TBH we had endless problems and I was forever writing to/phoning Staywell for replacement parts, repairs, general complaining purposes.
In the end we just never put another catflap in and do without one now as we got fed up of other cats coming in and spraying everywhere.
Our neighbour has one of the new expensive ones triggered by the cat's micro-chip. Shes been really pleased with it I think but they are not perfect as our cat managed to get through somehow.