Jekyll has a very simple, convention driven feel to it. Templating is based on Liquid which can be a limitation in some cases.
Middleman feels more like Rails for static sites. Out of the box templates are ERB, which is messier but more flexible than Liquid.
StaticGen.com is built with Middleman, and that made it easy to add a Github extension that would pull in all the repository statistics from Github during the build and make this available to the views.
Jekyll is more blog centric. Middleman is more generic. I have used Middleman for non-blog sites where you just need some simple functionality of templates and partials.
I'm wondering this as well. I've really enjoyed Jekyll in the past and the latest version of Jekyll has support for Sass and CoffeeScript compilation out of the box now (not that it was hard to add before, but nice to just have now). I would also like to know about what Middleman offers.
I must admit, one thing about Middleman that makes me roll my eyes is the use of "Hand-crafted" in the description of the project.
From Middleman's site: "Middleman ships with support for the ERb, Haml, Sass, Scss and CoffeeScript engines." I swap in a middleman-slim plugin, though.
So it seems that Middleman offers very little than the latest version of Jekyll, though I did read someone say Middleman is more focused on pages as opposed to Jekyll being very focused on a blog/posts.
I haven't used the latest version of Jekyll, but from having used older ones, I can say Middleman definitely feels more flexible to me, in terms of how its configured. See my comment elsewhere in this subthread for more color on that.
I've used both, and I got a lot more flexibility and familiarity out of Middleman. It's built as a web application framework that saves all its rendered pages as HTML, so it feels more like setting up a Sinatra app than anything else. I felt like I was fighting Jekyll and its conventions, but merely configuring Middleman and its plugins. Looking at the config file for my personal site, it's got blog stuff, layout variables, helpers, markdown customization, and an asset pipeline all set up with an easy DSL. [1] IOW: use Jekyll for simplicity, use Middleman for control.
A major strength of Middleman over most static site generators (including Jekyll until v2 launched recently) is the ability to generate large numbers of HTML pages from a single YAML (or JSON) array: the sort of thing you'd usually use a database for.
