If you run it in a VM (KVM/QEMU) you might need to set some CPU flags for the VM (otherwise GMP will assume your CPU has some instructions that it doesn't actually have in the VM): https://github.com/NixOS/nixos/issues/120#issuecomment-14776...

Other than that it works fairly well (in a VM), and you don't need to wait for it to compile all the packages like with Gentoo: if the build server has a binary then it will download that.