I use WordPress to develop easy-to-maintain, dynamic sites for clients and at my job at KHM Travel Group. I also used WordPress to develop personal/project sites while at Bowling Green State University.
I used CodeIgniter while working at KHM Travel Group. It was the system we used for an affiliate-based site called TravSearch, which I later replaced with a WordPress Multisite. I developed a .csv upload form to transfer all the information from the affiliate table of CodeIgniter to the user and options tables in WPMU.