I have been using WordPress mainly for building websites since 2007. WordPress is an open source software that allows developers like myself to easily customise the code. It has a great variety of functional plugins which can be used to enhance websites capabilities.
WordPress has a user friendly back-end admin area, which is easy to use for non-coders and coders alike. Any breaches of security in the code are usually found fairly early due to the global use of the software. Regular updates help with security and websites should update to the latest versions of the core templates to avoid these breaches.
I like WordPress because using their templates enables me to build websites a lot faster than from scratch and means I can keep my labour costs down for the customer.