One of the problems in using Apps Script is that you have to work in the Web page editor that Google provides. Now you can use a full featured IDE in the form of Eclipse.
The Google Plugin for Eclipse is what you need if you plan to build a bigger than average script, or if you want to work with other people. You can make use of the Eclipse editor to create your scripts and the plugin syncs the files with those on Google Drive. The editor has autocomplete and you can manage the files using whatever version control system you want to install into Eclipse.
The only downside is that the files have to be uploaded to Drive to run, and this means you don't get any help with running or debugging your application.
The addition of Eclipse as an IDE for Apps Script is a good thing, but it just demonstrates the simple fact that whenever a scripting language is added to a system it tends to be used far more than anyone ever imagined and, sooner of later, it starts to behave like a full language. Now all we need is a remote debugger.
To make use of the plugin all you need to do is install Eclipse 3.7 and use the Marketplace to download it. After this you can to sign in to Google and from here you can import your files from Google Drive. You can't use it to create a new project.
The plugin makes use of the Google Drive SDK and you can find out more about how it works in this video: