As of just writing this, Express 4.0 was released and there are points in there that no longer matter. So, this remains as a great >4.0 article.
Node.js is the red-hot new hotness! You can't throw a stick on the internet without hitting someone talking about Node. But why? For one, it's built on JavaScript which is completely ubiquitous. So, why not build a development stack and server on JavaScript? I would argue that the installation is almost painless while the terseness of the language is not.
While you can create apps 100% from Node.js, the Express framework is a great tool that helps you solve many standard problems without having to write boilerplate code.
Node.js is here and it's not going anywhere anytime soon. So if you are new to Node.js, Express, and even JavaScript in general, this is a great newb's step-by-step guide to get started.