• It is PHP, this might be a dealbreaker for some, but for me that is a huge plus. Primarily the advantages of this is that I can easily host my creations on basically any cheap web host, that there are plenty of libraries and resources out there to make my life easier and also that I know PHP. For the last reason I could have gone with Ruby on Rails or Java as well, but it put all Python frameworks out of the competition.
• CodeIgniter is very easy to install and as easy to deploy. All you need for things to play nicely is an Apache server, a MySQL database and a copy of the CodeIgniter files. Deploying and setting up things are the most boring thing when it comes to writing your own apps, so it is a must for me that it is a breeze, I just dont have the patience to deal with deployment problems.
• Great documentation, the CodeIgniter user guide is excellent. This is a huge difference from many other frameworks and platforms, especially the ones developed by an open source community (CodeIgniter is developed by Ellis Labs, the guys behind the Expression Engine blogging platform). The developer community is also very active and knowledgable, so what isn’t in the user guide is in the CodeIgniter forums.
• Finally, a framework that improves my productivity. Most frameworks tries to do to much and are so huge and rigid that there is a huge learning curve if you want to doing anything but a “Hello World” app. CodeIgniter helps me with the stuff I need help with and doesn’t meddle in the rest. There is no need to hack 10 plugins of different qualities together to get what I need (like in Drupal), and that just makes developing fun as it should be. It is also the first MVC (Model-View-Controller) framework that helps me organise my code in a good way, something that I usually suck at otherwise.

In short I recommend that anybody that knows PHP and want a light weight, good framework checks out CodeIgniter. A good place to start are the CodeIgniter video tutorials, and if you get a bit deeper into things Elliot Haughin has a great blog that often covers CodeIgniter and he also have some great libraries that are well worth looking at (CodeIgniter libs for Twitter, Flickr and Akismet for example). Another great resource is the blog of Derek Allard, Technology Architect at Ellis Labs.