This is now in the early stages of UK release - it isn't quite free to join, you have to pay £6.99 for the sim and postage, and the service they offer by default is a paid one. But if you select the free one and keep to their limits - 200mb, 200 minutes, and 200 texts per month - you don't pay for service. There's one thing to watch out for, they charge you a fiver top-up fee if you are close to the limit, but that can be switched off, and I will probably do so. They also have you receiving offers from themselves and other advertisers by default, I'm switching that off too.
Once the SIM arrives I'll see how it goes and post about it.