I got one from Age UK for my DH several years ago, which which seemed fine. He has never needed to use it for an emergency but they liked him to test the buttons every couple of months, so he became very comfortable with using it. It came with an outside key press, which is useful in itself (with an easy PIN, he can let himself in if he loses his keys).
BUT. The problem is that Age UK sold the business to someone else, who sold it to someone else, who sold it... Well, you get the idea. The service has gradually gone down the pan with each successive owner. Now, the response time is dreadful - up to 20 minutes of electronic beeping, cutting out, re-dialing and more beeping. If you factor in the time they'd take to answer, speak to him and call out the emergency services, plus the time they would take to retrieve the key from the press, the waiting time becomes more or less open-ended.
Sick of the whole rigmarole (for which we paid quite a lot), I decided to buy him a very simple mobile phone, for the sole purpose of being able to dial 999. The phone is a lot cheaper, very much faster and it gives us both peace of mind. I'd never get another "personal alarm" unless it came with a guarantee that someone will answer straight away and he'd get assistance within a sensible time.