I have used it a bunch of times. I am currently working on one here...
http://www.thegraphicdesignresource.com/interpreter for a woman who is an interpreter obviously.
It is not too hard to make validate, but, you have to mess with some of the code that generates errors. The core is not bad but some of the extensions are.
I have never had much luck with any seo hacks from their site. While the url's it outputs are a total mess, I have never had any seo issues though.
Get your extensions here:
http://extensions.joomla.org/
I have thus far installed community builder, some newsletter mod, jobline,google translation, and something else I think.
I can likely answer specific questions, feel free to shoot me a note.
Nigel