I’m going to jump on this ancient thread to share my own (mostly negative) experiences (although these were with an upgrade to 20.03, rather than a fresh install).
The short of it is: 2 major things are broken (screen dimensions, copy-pasting) which makes this a huge pain. And I haven’t even done a cursory check of other things that might be broken yet. Links below.
It would seem to me that NixOS not working (properly) inside a VirtualBox VM is a huge blocker for further adoption. As per the manual, which states “Installing NixOS into a VirtualBox guest is convenient for users who want to try NixOS without installing it on bare metal.” I find this a pity because I sincerely hope that NixOS will become more successful than it already is. The fact that I’m encountering issues this large 3 months into the release (it’s june now, I’m talking about 20.03) also makes me wonder whether I’m somewhat unique in running NixOS inside VirtualBox (I’d prefer not to be).
Now on the subject of doing a fresh install, I actually kept a log when doing this 3 years ago, and at least at that point in time (17.03) things still worked with the standard ISO. (My notes specifically mention taking the standard ISO rather than the pre-made VirtualBox image)