A year ago I wrote a post about the hottest Content Management System that everyone was using to build websites, WordPress.
I could take that article and change "WordPress" to "Joomla" and it would be 99% accurate, today. For Joomla is the CMS du Jour - the Content Management System of the Day.
Probably in another year it'll be something else.
Anyway, at the risk of incurring the wrath of people who are in love with their Joomla, you should know the facts:
Joomla, with the addition of a plug-in, sh404SEF, can be SEO'd. It takes twice as long to do everything, but you can do it.
It has the liabilities of other CMS systems: it is hard to make your site look really good and layout issues are common. You can easily change things you shouldn't really be changing all the time (like the menu). You have a dynamically generated site, meaning, you are making the server work a lot harder.
Yes, I have seen really good, professionally done Joomla sites, though they are rare. I have seen a Joomla site with decent SEO. One.