I enjoy using betas on my main devices. I’ve been doing it with every beta since iOS 3.
15’s been incredibly stable for me. I had one somewhat critical issue with Beta 1, where iCloud Private Relay would just kill all network traffic, but since then the only issues I get are visual, with the occasional hang up. Sometimes Pokémon GO freaks out and I have to delete and reinstall it, but other than that no issues with third party apps.
I really like iOS 15. I’m a big fan of the new Safari (I find it much easier to use one handed), the new Maps interface I’m enjoying, and I find Focus modes to be super powerful.