WordPress

Wordpress is well supported and well established. The popularity for the project is extremely high, which is great for finding plugins and addons, and getting support for just about anything you can dream up. However, a mediocre API encourages too much randomness in plugins allowing for security and major performance issues with many plugins. Even many popular plugins have major performance implications. A caching system is a must with a Wordpress system, staying up to date is also crucial as there are numerous security vulnerabilities appearing often.

I like the cool admin interface, the API and other things, but it's really buggy.
And it's PHP.

We use WordPress on CMS Critic.com. It's a great piece of software and still one of the best blogging platforms to date. Definitely rates high in my books and would highly recommend it as both a CMS and Blog

I have written several blogs now, I started on blogger and did not like their approach, I tried several others and did not find what I wanted. Along came Wordpress 2.3, I have been a fan ever since. Very usable, and easily customized, hundreds of different templates and things to add into your blog. Security can be done as there are some security conscious folks that have now developed some very nice widgets to help in the security aspect. Easy to install, upgrade and change as needed. Excellent product, and yes it proves you can get something for 0 cost...except time.

From security standpoint there are not many choices worse than WordPress.

I see a lot of very nice WordPress blogs. Mine was never so nice, as I found it to be considerably more work than I expected to do such things as swap in a new skin. The WP version upgrade process totally whipped me, though, and I am now no longer a WP user.