Installed it straight after unboxing, with Play services. Ended up using it pretty much like any Android phone. Installation is simple using the web installer. On recent versions, even Android Auto works, so the only thing you're really giving up is NFC payments. Some banking apps may don't work, but I'm lucky (or rather not unlucky) that the ones I use do. I believe those rare apps are somewhat lazily developed, and rely / trust on Google to do security for them.

Some months later, I went back to the stock ROM, mostly for comparison. Stock Pixel OS has a lot of appealing features, but most of those are just "nice to have" things. Stayed on stock for a few months, but the plethora of obscure Google "privacy settings" put me back to GrapheneOS, and finally off Google. Reverting to stock was also simple, just as easy as flashing GrapheneOS.

Now I don't have Play services at all anymore, and have cleared most Google services (gmail, photos, drive...) so at least not much new data will go there. I do use Google Camera, and have Photos installed since I think the post-processing happens in Photos. Both have network permission denied, which is one of the nicest added features of GrapheneOS. The stock GOS camera is OK, but that's one thing I think Google does better, though this is a subjective thing.

The only thing I kind of miss is Google's find my phone stuff. Even though it's quite invasive, I have needed it once and it resulted in me getting a lost phone back. A simple solution is not to lose your phone.

Apart from the per-app network permission, another really nice feature in GrapheneOS are the settings to toggle WiFi and Bluetooth off automatically. Why these are not in any "official" ROM tells a tall tale about how much they care about your privacy. The auto reboot if not unlocked in a while also brings some assurance regarding losing your phone, at least the storage will automatically back in encrypted unlocked state.

Vanadium might be the best browser I know for Android. Pretty much Chrome without all the things that make Chrome one of the worst browsers. Vanadium's point is security, privacy (e.g. adblockers) is not the main focus. I'm not sure if there actually even is adblock features bundled nowadays.

If you want all the nice modern bells and whistles, stay on some other OS. If the benefits above appeal to you, there's really not much you give up in the end with GrapheneOS. It requires a bit more technical mindset, but not really even technical know-how. I haven't noticed bugs or broken stuff anywhere, with or without Play services. Android Auto (requires Play services) gets stuck sometimes, but that may also be my low-tier car too.

The "sandboxed" Google Play refers to the apps running as user installed apps vs the system-wide root-access-to-everything apps they are on stock. The same limitations you can apply to any other app you install apply to GSF apps too. So even if you install Play services, you are severely limiting the scope of data Google gets from you. It's a solid middle ground between full degoogling and stock OS.

I'm not even an Android app developer, and will gladly admit technical mistakes. If you want something negative, the vocal minority of GOS users is really vocal and really full of themselves.