Plan9 (as well as its spiritual successor, 9Front) represent a radical departure from what most folks tend to think of as the normal paradigms of an operating system.
I've poked at Plan9 for a few years, but never made the leap to using it full time – my day job requires that I do a lot of web development, and Plan9 is more well suited for lower level programming – mostly in it's dialect of C.
The 9front FQAs recommend not installing 9front on Virtualbox since the install breaks whenever Virtualbox updates. This hasn't been my experience, and because I'm not doing any "serious" computing under 9front on Virtualbox it isn't the end of the world if I need to scrap the VM and start fresh.
That said, here are my notes on setting up 9front on Virtualbox:
Note that on my 24 inch (3840 x 2160) monitor the virtualized desktop of 9front is teeeeeeny tiny. My solution for this has been to move closer to the screen and to enable "scaled mode" on Virtualbox. Not great, but combine they work for my purposes.
Also of note – core to Plan9 and 9front's UX is a 3 button mouse. Without one using the system is difficult to impossible to navigate. I've made due with a track pad, but a bona fide 3 button clicky mouse is ideal.
If you are interested in getting your feet wet with Plan9 but don't want to stand up an instance, Plan9 from User Space is the easiest way to dive right in. Purists will poo-poo this, but I think it is as solid a starting point as any, especially since it eliminates the complexity of standing up a virtual machine, or installing on to bare metal.
Similar to Plan9port is 9vx. It runs a more or less complete instance of Plan9 as an application in user space…I've had less luck getting this to work lately, though.