Many people like Mobian. It has a very nice UI (Purism's Phosh) and the full GNU/Linux under the hood.
I personally would recommend to try Sxmo: https://forum.pine64.org/showthread.php?tid=9913. It is unbelievably fast, capable and has a cool geeky UI.
> Sxmo, or Simple X Mobile, is a collection of simple and suckless X programs and scripts used together to create a fully functional mobile UI adhering to the Unix philosophy for the Pinephone.
It reminds me of working with text-based user interface in the terminal, with intuitive window manager. I love it!
That's also the reason why I almost immediately threw UBPorts off my PP UBPorts CE. You can't do anything like you would it in Linux. For example, mounting the SD card automatically on boot with fstab wasn't possible as it always reset itself to default values. The root partition is also read only. No thanks.
For trying other OSs. Just flash them to an SD card and put them in the phone. It'll boot from the SD card before booting from the eMMC.
The physical switches are nice, but you still need to have some decent functionality to make it a daily driver. Sadly, it is not there yet.
Since we are talking about it, if someone can recommend good alternative to Ubuntu for my UBports edition, that would be awesome.
I am slowly preparing to try other OSs on Pinephone, but since it is not super easy process ( and since you have full access, you can easily brick your phone ), I have thus far spent more time learning about it than just trying to jump on one of the other OSs.