I've only just used WordPress commercially to develop and maintain an e-commerce site and wow, I must have saved at least 100 hours of development by using it.
I've tried creating a CMS by myself using CodeIgniter's libraries and only accomplished the basic CRUD functionality, which means I have so much respect for WordPress and how much it offers me in a package.
Okay, so I'm comparing creating a CMS from scratch to using WordPress so of course WordPress is amazing, but it's not just that. The amount of functionality available in themes and plugins gave me a lot of confidence as to what was realistically achievable. My hand was held :)
Without WordPress my e-commerce project would never exist because I wouldn't have been able to spend as little time developing it as I have now.
Thanks to the community (I was going to list you all out but I would only end up excluding people).