Recent estimates is that wordpress powers over 20% of all websites. Thats a growing number and for good reason.
A few years back, wordpress departed from a bloggers tool, on its way to being a platform for all sorts of websites. Along the way, 10s of thousands of plug-ins have been developed to achieve that goal. Everything from SEO to shopping carts are available, so as paid additions, most are free. WordPress is a open source application, meaning it has been developed by the web community at large, and additions to the software are free to develop. Installation of wordpress is free, and there are uncountable templets pre designed to customize the look and functionality of the site.
There are two main draw back to a wordpress install. First is that it is a database driven platform, meaning there is a lot of coding on each page and that when accessing a database to display the site, there is more chances for things to go wrong. Database sites are also slower in displaying due to the amount of data need to display each page.
The second issue is with hacking. Since its a popular platform, its a target to hackers, so extra precautions need to be taken in securing the site.
With those things aside, its a great platform when editing capability is needed on the client side. The admin interface is easy to work and there are many tutorials to help.