It's cool when it works. Extremely frustrating when it gets in your way, I can't be bothered to learn its god awful configuration language just because I need to use a customized version of some software. Guix is probably not nearly as painful in this regard, since it has sane configuration, but still compared to something as frictionless as Void I'd rather never touch it at all.
Also troublesome if you're not careful is that sometimes, you may end up with software that depends on data, which Nix does not handle, and can have nasty consequences when Nix rebuilds the system without you asking it to when it thinks the data is inconsistent (had a close call with a Nextcloud instance because of this).