- ~CON: It's not as polished experience as Ubuntu on "first install", i.e. "end user first" or "Windows-like". But based on other comments here, I assume if you're willing to try Arch, you agree for some tweaking. Please note I've never used Arch, so I can't more precisely compare the level of tweak-ness required; what I can say for sure that it's certainly easier than what was required in '90s (at least because we have teh internets now; and esp. the ArchWiki + askubuntu...) But again, I started my post with "if you're willing to consider Arch", so in such case I assume this point is a non-really-CON, as you're already agreeing to take this cost.
- CON: you have to learn a new language (Nix). To sweeten the deal, IIUC it's one of the few truly purely functional languages around. No IO monads or whatsit. As a result, you may (um; will have to...) learn some interesting functional tricks you never imagined may exist. Note also, that some advanced usage ideas are spread over a few blogs (see esp. the "Nix pills" series), and also in inline comments in the Nix standard library source code.
- SUPER-PRO: I found out that NixOS is a hacker's dream OS. Nix's core idea is that removing a package from your OS nukes it clean, leaving absolutely no trace it ever existed. As a result, you practically don't fear any changes in even most sophisticated internals of the OS. Want to change a kernel compilation flag? Meh, let's just try this one, bam, compile, reboot! Oh, it hanged during boot? Pfff, reboot again, select previous version in GRUB, and we're back! [Um; I mean, as long as you haven't burnt your hardware ;) you know, Linux is powerful :)] Also, part of this is in the fact, that all of your OS config is described in a single file, so you can control everything from single central position, and VCS it trivially.
I've already sent a couple PRs based on this ease of hacking and tweaking, namely to neovim and systemd-bootchart. I'm also trying to write my own series of blogposts documenting this (while it's still fresh in memory); but, eh, you know, a bit too many side projects... not to mention even some of this weird "real life" thing people are talking about so often...