Oh, hi! Guess it's time to resurrect a dead thread! :D

I'm currently on NixOS* and yes, it's stable, but there are quite a few gotchas that you won't know about until you run into them:

• GPU drivers are not quite as plug'n'play as the documentation makes it seem, for example. There are settings for selecting to use the AMDPGU driver for the system, but that won't actually add the driver to the kernel modules - that's a separate setting that's not mentioned in that part of the docs. There are other little things like that which gets you into the habit of checking the source of settings before enabling them (which, admittedly, is not a bad practice).
• Home Manager is great, but while it technically allows you to have the system follow the stable and your user-space follow unstable, that won't actually work with QT applications, as both the stable and unstable libraries will be available in the search paths and QT applications just can't deal with mixed versions like that. I recommend just sticking with either stable or unstable and maybe pull a specific package or two in from the other channel if needed.
• There are no real "official" extra repositories. If you need something that's not in the main channel, expect to be searching Github and Gitlab or start writing the support for it yourself - which is usually pretty easy though. There is a Nix User Repository, but I haven't had any use for it yet and, again, it's really just "someone else's repo".
• I like this part, but you're going to (or should, at least) be using the Nix language for practically everything you ever change on your system; it's a good thing in my opinion, but it's certainly not for everyone.

Bottom line: if you like the idea of managing your system in a functional programming language, then I really do recommend NixOS :) If you just want a stable base without too many odd programs installed on top, then a basic NixOS install also isn't much of a headache while still staying on the surface of writing Nix code - just beware that the temptation to "nixify all the things" is very real ;)

* my Arch install borked itself for the second time this year by doing an upgrade, running out of disk space halfway through and leaving the system unusable without some rescue-boot operations; suddenly Nix sounded very attractive!