Their top up mechanism has worked every time I've tried it. I've done manually, not auto-renew.

Also their customer service has generally been very good. Email support has been quick and helpful. I had some issues with a couple of older no expiry SIMs and they just replaced them with working versions. Much better than "are you sure you have an unlocked phone, have you tried a factory reset, let us re-provision at our end within the next few hours, etc."