And they have great clients for Windows, OS X and iOS. I've found a few others that are just as leak-free.[0] However, the data there are old, and just about all VPN services have improved their clients. What's most relevant about the site is the testing protocol. There's more about that in an IVPN guide.[1]
I also recommend AirVPN, Mullvad and PIA. But not necessarily for their clients. I mean, IVPN doesn't have a custom Linux client. So in many cases, you need firewall rules. And you need to make sure that you're not using an ISP-assigned DNS server with the VPN.
1) https://www.ivpn.net/privacy-guides/how-to-perform-a-vpn-lea...
I used PIA for a couple of years without issue, but then it went into some kind of decline for me, always driving network traffic to zero after a few hours. After changing hardware and reinstalling the OS with no effect, I finally tried AirVPN and things went back to normal. AirVPN is a bit more expensive, but their client is light years ahead of the PIA client.
The only step beyond this that I have seen is a recommendation to use OpenBSD as a firewall in a virtual machine.
But. It's basically what I described. For public VPN network, just use the default (all output, only established input). For private LAN, deny all output and input, and allow output to selected IP addresses (VPN and DNS servers).
Perhaps something like this can be scripted; if it becomes polished enough it could be recommended as a part of every VPN setup.
That's why you generally ignore online reviews.