It's mostly a reliable day to day phone. Rough edges I've seen:

- Occasional "X app stopped unexpectedly" messages.

- Font sizing is really weird on the built-in email app (a K9 Mail reskin)

- System webview sometimes (maybe once a week?) starts rendering blank white pages until I reboot the phone.

FWIW, I have experienced all those things with different Android versions, and mainline K9.

I have been using it for I think over a year now as my only phone, on a Samsung Galaxy S9+. It's honestly not much different from Android (although a little out of date), and the only real issue I have is that installing software is a little inconvenient. (The built in app store doesn't work reliably.)

I assume in-app purchases are also a no go, right?

Right. It's fine for me since I use free software as much as possible anyway. The only applications I've sideloaded are WeChat, Teams, Discord, Target, Instagram, and WhatsApp, all of which work fine.

Edit: and Google Maps. The built-in mapping software doesn't have good enough traffic estimation.