I've been very happy to see that, for the most part, the evangelism around Haml has died down over the past year or so -- it felt like you couldn't go anywhere in 2010 without getting slapped in the face with significant whitespace. It still crops up every once in a while, though, so I thought I'd add my ever-welcome $0.02 and explain why I don't use it.
- HTML isn't broken. Sure, it takes more keystrokes, but there are no faults in HTML that are fixed by Haml (unlike Sass, which actually does fix problems in CSS -- and can I just cheer the separation of Sass into its own gem?).
- Significant horizontal whitespace becomes a gigantic problem when you have more than one screen of content. Even relatively simple web pages can run to multiple pages of markup, invalidating the "indentation as visible structure" argument.
- Haml implicitly promotes a
<div>-heavy markup structure -- it's just too easy to go
<div>-crazy with Haml, instead of the slimmer, more semantic markup style that I prefer.
Of course, your mileage may vary. Do what you want, but please stop accosting those of us who choose something else.
Edited: Haml isn't an acronym, so all of my "HAML"s were incorrect. Sorry!