for me it is as many have said. Light weight, feature rich, simple to use, simple to extend, small footprint, excellent community, excellent third party libraries, the best documentation i have seen in any framework.
i have been using CI since about 2 months or so after 1.7.2 was released and have not looked back.
CI is not the end all and be all of PHP frameworks, it is not perfect. But it is perfect for me.
CI is so easy to turn into what ever web app you need it is crazy.
"I reject your reality and substitute my own" - Adam Savage, M5 Inc.