For me 1Password's client, not the backend, is the winning story. I'm sure someone could build a great client and browser plugins for Vault (or any other backend) that could compete with 1Password, but right now, I've seen nothing. (LastPass in some ways is close, in other ways, not so much)
I'm guessing you and GP are using macOS and probably iOS? 1password lacks a Linux client and my experiments with both the Linux Chrome extension and the Windows client were not great.
Yes. I've used the Windows client, and I'll admit it's not bad, but not great. The Chrome extension (actually works on any version of Chrome; I'm running it on OSX in addition to the extensions that plugs into native version) is a bit lacking.
Part of what works great about it is the TouchID/FaceID integration on both iOS and OSX.