All the packages I need have been part of nixpkgs, except for node packages from npm. For those, I run npm2nix to generate nix expressions from the package.json files. That's occasionally tricky when npm packages do "clever" things that don't work when translated to nix. Generally, though, npm2nix works pretty well.

I also have about 3K lines of nix code for packaging my apps, configuration, cluster definitions for nixops etc. There's a bit of a learning curve, but I find it really pleasant, now that I have the hang of it.