Compared to Jekyll (Hakyll's obvious namesake), it seems to take what is a remarkably simple system (ie. one that puts your focus mostly on the HTML, CSS and JS and does all the hard work for you), and replaces it with something that you need to spend time actually learning to be able to even compile.
While Jekyll offers convention over configuration, this seems to go the other way. For a static website I'd rather focus on the HTML and CSS than a DSL that compiles it in a certain way.
While Jekyll offers convention over configuration, it is not that configurable at all. Hakyll is definitely harder to use, but more flexible as well. For example, it is possible to write a tool which mimics Jekyll, built upon Hakyll:
Hakyll is an excellent and active project. I used it for the Solarized site and learned a lot about Haskell, though like many Haskell projects (xmonad) you can still get a lot out of it without being a real Haskell pro.
One neat trick I used was to scrub my pages using sed first (there's probably a better way to do this with just Haskell) so that I can use the exact same markdown and links on both github and my own server, and the images and download work on both:
I am using MiddleMan, but I have pandoc parse the markdown first then pass the HTML over to MiddleMan. Pandoc is a godsend for academic writing, although I'm also using MultiMarkDown for some of the files due to the way it handles tables.
The author of Pandoc has his own Haskell static site generator, although to me it lacks the elegance of Pandoc.
I really like the ethos of Hakyll, and have tried using it in the past but couldn't wrap my head around it.
I feel the tutorials would benefit from something more for someone who is also new to Haskell and Markdown, as well.
While Jekyll offers convention over configuration, this seems to go the other way. For a static website I'd rather focus on the HTML and CSS than a DSL that compiles it in a certain way.