I really love using WordPress, and it's the best content management system I've experienced. Here are five reasons why:
1) Easy to use. It's more intuitive than Drupal, WebVanta, and other content management systems. I can build sites out quickly, and even update sites on my phone on the go if I need to (like fix a spelling error immediately).
2) Least expensive way to a great website. So many free and low-cost WP themes are offered with so much functionality. One-click WordPress installations are available on many website hosts. You can be up and running a solid website in a few hours for less than $25.
3) WordCamps. This is most fantastic way to meet people and learn more about WordPress. Highly recommend. Find a WordCamp near you. I'll be speaking at the upcoming WordCamp in Raleigh, NC, USA in November where you can learn how to get your website up and running.
4) WordPress community. I had a stranger in a WordPress forum solve a problem for me in minutes that would have taken me at least 12 hours to figure out on my own. WordPress users are generous with their information and show a true willingness to help others.
5) Clients can figure WP out - on their own, even if they forget everything I've told them or written out in a training document, even if they don't consult the many good sources for learning WP available online. My clients get busy and forget about their websites for way too long, but then they can come back to update them and quickly remember how WordPress works. Hmm, basically this is #1 repeated for clients. Really the ease of use is the most compelling reason to choose WordPress.
If you don't use WordPress yet, tell me why! What content management system DO you use?
(BTW I have no affiliation whatsoever with WordPress except as a very satisfied user.)