CodeIgniter is probably the best and “least fatty” frameworks I’ve used in a long time. CakePHP has its charms, and I’m sure Symfony has its good points too (though I’m not familiar with them) – but CodeIgniter gives you just what you need, and lets you interject the rest.
This site (www.tomduff.net) used to be run by a shoddy, horribly hacked together CMS that I wrote when I was first starting out with PHP. Instead of looking into a framework to ease my work, I chose to build on top of the code I had. In hindsight, I suppose this helped me figure out the intricacies of the language, and somewhat good practices, but for such a large project, if you keep building on top of what you’ve got, you tend to pigeonhole yourself at one point or another, and wind up hitting a wall for new features.
I think this whole thing started when I began looking into Ruby on Rails to fix the issues I had, but what I really needed was the cleanliness an MVC framework could give me, while at the same time, allowing me to use a language I already knew. Ruby isn’t that hard to learn, but I use PHP more, so I was better off with an MVC framework written in, and for, PHP.