I use it on the client right now with Backbone.js, but I'd like to use it with node.js also.
There are directions on the requirejs homepage that I found pretty useful: http://requirejs.org/docs/node.html
I also set up some examples here: https://github.com/rgarcia/node-requirejs-examples
The basic idea is that you use it just like on the client side, but for npm modules and built-in node modules, you don't use the relative path, but rather the module name. For all of your custom modules you use the relative path.
This recipe can be found in it's original form on Stack Over Flow.