Inspired by watching TJ Hollawaychuk's supremely awesome screencast about using route-specific middleware at http://expressjs.com/screencasts.html, I threw together a simple use case for myself: pagination. While pagination might not be an obvious choice for middleware, it works here because it doesn't need to know anything about the route that's using it, though in this case it may take advantage of some passed in variables if they exist!
Here is a partial using EJS syntax that will display some previous / next arrows wherever you choose to place them. Notice I'm using locals.prev and locals.next here, which works, becuse I'm not sure those variables will even exist. In these cases you access them as properties off of the locals object in the view, which means you don't have to do this if (typeof prev !== "undefined" && prev), which is a little more verbose if you ask me:
I liked how easy the middleware approach was over having specific pagination logic in each route that used pagination. Express is fantastic for letting me send in multiple functions that I can chain together by calling next(). In this example, without using the middleware I would have had to wrap each paginated function in the count() database call, which would give me an un-neccessarily long callback chain. I prefer the simple middleware approach allowed by express.