I love Wordpress because it makes developing websites very easy and scaleable. I create very custom designs (usually with Thesis 2.0 or a ThemeForest theme which I heavily modify using CSS and/or PHP), and it allows for me to create just about any website in 10-14 days. Some large projects take a little bit longer, but we have never had a web design turnaround longer than 45 days.
Our company's fast turn around time has won us loads of business because most web design companies in our area tell their clients to expect the design/development process to take at least 3-6 months (I have heard quotes of up to 18 months!). Once we agree in principle to work with a company, we usually give them 2-4 styles (psd's that take about 20 minutes each) to choose from and tell them to expect the site to be ready for review in 2-3 weeks.
I have used several other CMS's (Joomla, Pyro, etc), but I strongly prefer Wordpress. It's not because I am lazy or because no one in our company knows how to code; as a company, we are best at sending targeted traffic to a website, and we like to finish up the design and begin sending traffic as soon as possible.
The speed at which Wordpress allows us to crank out new websites has presented us with a lot of opportunities and turned into a lot of direct revenue.