Depends on the type of documentation, but I'm a fan of LaTeX since there are so many tex2____ programs out there that'll convert LaTeX to whatever format you want.
The learning curve is steeper but the flexibility is high.
The learning curve is steeper but the flexibility is high.
Bonus for being able to render mathematics in your docs.
Edit -- Also, I agree with you about the learning curve. Although it's very easy to get good-looking docs if you use one of the canned documentclasses and built-in commands, once you start to stray from those you're in deep water very quickly.