I'm using GrapheneOS on a second device for various reasons. The biggest issue for me is that not all apps work / run. However, I have limited app requirements, so that is fine. If you want to run all social networks, Uber, Lyft, and so on, there might be the one or other that doesn't work (I didn't try them all). However, you can always use the mobile web offering I guess.
In terms of classical smartphone features, I know what I don't get out of the box due to the lack of Google Services (Assistant, Picture Sync, etc). That wasn't an issue for me as it is a secondary device.
GrapheneOS now has https://grapheneos.org/usage#sandboxed-play-services providing the ability to install Play services in a sandbox. The core functionality is already working in the Stable/Beta channel releases. You can install it in a dedicated profile to avoid apps in other profiles being able to use it.
GrapheneOS is fine with people using Google apps and services but not integrated into the OS and they should be on the same level as other apps and services without any special privileges/access. We're working on making this a reality. Google could implement the fallback code paths we're providing for Play services themselves. All we're doing is teaching it to do what it should already know how to do. Perhaps a regulator can force them to unbundle their services and make them usable anywhere.
Not the person you asked, but my banking app works but occasionally will crash when I go to certain parts of the app. I'm not sure why.
Other than that Discord, MS Teams, and ProtonMail all work fine with the exception of push notifications (I disable those anyways, so this isn't a concern of mine).
In terms of classical smartphone features, I know what I don't get out of the box due to the lack of Google Services (Assistant, Picture Sync, etc). That wasn't an issue for me as it is a secondary device.