I hate to just add a plug for myself, but I use WordPress as a cms on my own site - http://dkuntz2.com.
Update: I no longer use WordPress.
While it's still mostly a blog, I do have a portfolio bit built in.
The main reasons I use WordPress over another cms are that I know WordPress, it has a huge community dedicated to supporting it, it's easy for someone who's never used it before to start and while it may technically be a hack, I've got my portfolio setup using post categories (I could set it up with custom taxonomies, but I use categories because I used it before taxonomies were out).
I'm not sure if other cms' have that ability, to keep things dynamic like a portfolio but still have a blog without using the same hack.
I use WordPress with clients because it's easier for them to use than Durpal. I would never use Joomla simply because it's really bloated and slow, and the admin area doesn't work very well. It's also nice that lots of people have written getting started guides for WordPress, so that I don't have to write a getting started guide myself, just a guide to how their site works.