I love Middleman. It's simple, powerful, and fast. I've been playing with Rails and Rails prototyping for a while, mostly with Serve—and with static site generators like Jekyll and Octopress for about just as long. I think Middleman is the best of all of them.
Middleman is built on Sinatra. As such, it gets to make use of all the niceties of modern web development in Ruby. Rails conventions, Padrino template helpers, templating in just about anything you could want (ERB, HAML, Slim, and a ton more, preprocessing (Sass, CoffeeScript, etc.), minification, concatenation, dependency management, LiveReload, and so much more.
I think hosting static sites just makes so much sense. Fast, secure, cacheable, fit for CDNs, what's not to like? I've built a couple project templates that I use here and there. Both have layout schemes based on the great Frameless approach to adaptive grids. There's a basic site template, and a basic blog template which makes use of the blogging extension, and also powers this site.
This is a great time to be building websites.