Very easy.
git clone git://github.com/NixOS/nixpkgs.git
cd nixpkgs
nix-build -A packagename
Building the dependencies is completely automated. For more information, see the nixpkgs contributor's guide: http://nixos.org/nixpkgs/manual/Note that the above command will simply download cached binaries where they are available, but if you make any changes to the build scripts nix will notice rebuild locally instead. This gives you the benefits of both source based and binary based package managers.
Building packages on Debian like systems is a huge pain. Arch is much better but I need Debian/Ubuntu-like reliability.