I must admit I do an awful lot of work with WordPress. Like most software it's a bloated mess in places with lots of inconsistencies, although pretty easy to work with once you understand it.
From experience the main issue most people seem to have is that they install two dozen third-party plugins and then wonder why everything goes wrong or why they got hacked. We build some pretty nice websites with WordPress and never use more than 2-3 third-party plugins.
For run of the mill content management I would personally stick with WordPress. I'd only use CodeIgniter if I had either some very specialist requirements, or very basic CMS requirements. For anything in-between I don't see the point in re-inventing the wheel. That's not a criticism of CodeIgniter. For me, WordPress is a CMS (or a CMS framework, depending on your point of view and what you're doing), whereas CodeIgniter is a framework to build something from the ground up. But re-building WordPress from the ground up probably isn't a viable use of anyone's time.
