I found it interesting when I recently discovered people converting from Joomla to Wordpress.
The attached graphic from Google Trends shows that it's not any sort of mass exodus, but it is happening frequently enough to be noticeable.
Wordpress is inarguably less feature-filled than Joomla. Migrating a web site from Joomla's system of modules and plugins to Wordpress's system of widgets and plugins is like taking a step backwards in software architecture.
Still, it appears that many people do not need what Joomla is providing -- and are happy to shed Joomla's frustrating UI and layers of seemingly unnecessary complexity.
There are tools available to make this migration possible and reasonably painless.
I am not looking at migrating today, but who knows what the future will bring?
Here are a couple of the better pages I found on migrating from Joomla to Wordpress: