In my opinion Slim is superior to Haml. I used to love Haml, but Slim takes it to the next level. A syntax with less control characters and a higher performance [1] made me switch.
I still haven't heard a single argument for writing pure HTML instead of Haml/Slim. Even for static HTML I use Slim.
Slim also supports HTML streaming. Not sure if Haml have fixed this yet.
Why Haml at all? You will be changing all of your html in a browser anyways. Why not write html/erb?
I know this would become a debate, haha. HAML has definitely disadvantages, ERB is indeed nicer if you need to write a lot of nested tags, or stuff like strong, small, etc. Still I've found it's more pleasant to work with... But that's just personal taste and I should probably make it optional.
Coffeescript is really nice, though it can sometimes be a pain to debug. Haml is more succinct than HTML and I like the forced tab alignment. The problem is I almost never write view code that does not need to immediately be debugged in the browser. So I have to pay a mental tax to transform it nearly every time I write it.
You can mix them.
%p Here is a Haml paragraph with a <strong>bold</strong> word.
Personal preference, but I find haml infinitely more productive and easier to manage.