I’ve had one for a couple of years. I got mine for about £12 but it’s the ‘holder’ that actually costs the money. The SIM sized cards are pennies. I mainly used it because Barclays didn’t support Apple pay and still don’t do android pay (one of the reasons I got monzo) and it was pretty good. I had the one that went on a watch strap. They’ve recently moved over to pingit. This can all be used with any bank account, the service itself happens to be run by Barclays. It’s basically just a top-up account which you can set to happen automatically. Imagine an oyster card that can be used elsewhere.
I now have a smartwatch with Google pay so don’t need it anymore