[vertex-four]

It's generally well-supported enough so long as you don't need the latest and greatest packages within about a month of their release. They're having issues with their continuous integration system (the box it's running on isn't powerful enough and the project doesn't have money to get a new one), and there's occasionally breaking errors in important packages meaning a new version of the package repository doesn't get rolled out for a while, even if you're not using those packages.

Note that building from source is exactly the same process as building from binary - i.e. "nix-env -i firefox" will try to install from "cache" (the output of the continuous integration system's build process) and if it can't find something, build from source. Most important things are in the cache, some things aren't (mostly obscure packages and things with no-redistribution licenses), but in general it works out well.