Yep. The cool thing is that installing a binary version of a source package is totally transparent from Nix's point of view—it's not a different entity, it's solely a matter of caching. If you're building a package that somebody else built and uploaded to a cache that you can access, you'll download the binary instead. There's a public cache, so when you use a standard package set as your base, you'll be getting most things as binaries. But if you make some changes or something else breaks and binaries aren't available, it falls back to building from source transparently—sometimes this can take a loooong time, but it usually doesn't, and I'd rather have the option to do that than not :).

The other bonus is that it takes very little effort to share your own binary cache—either privately (so that you or your company only need to build something once) or publicly (so that people installing your open source tool don't have to build from source). There's even a really well-designed commercial service called [Cachix][1] that provides a totally turn-key binary cache, including a free tier for open source projects.

[1]: https://www.cachix.org/

I use this for building system. Instead of Cachix I just set it up as an S3 bucket (it's a bit involving but also works). I needed to tweak some things but ultimately arrived to satisfying results.

The nice thing is that it sped up building by 50% and also extra bonus with things is that with merges, if files didn't change nix will finish building instantaneously.

This is similar with nix-shell, where for example with python you enter "virtualenv" like environment almost instantly without downloading dependencies over and over. Yes, you could get close to do that with CI by using some caching, but this is out of the box, due to nix understanding dependencies.

Also another great thing is that so far, I never needed to purge cache, which often happens with normal CI caching.

my understanding is that most nix/guix users don't compile (most) packages themselves but download them from a binary cache. why not would be for obvious security reasons but at least on the guix side there's a subcommand to compare build hashes from various mirrors you trust: https://guix.gnu.org/manual/en/html_node/Invoking-guix-chall...