its the lightest-weight (edit: popular) framework and least restrictive. very bare-bones by default. plus, the docs are great.
if you feel like subsequently moving to another php framework from there, the principles are very similar and the transition would be easy.
i'm coming at it from a perspective of getting this dude up and running quickly on something good, which i think is easier with CI.
http://codeigniter.com/user_guide/overview/at_a_glance.html
which means they've missed all the performance and security improvements in PHP 5 (like applying filters to user input), not to mention that PHP 4 is end of life now.
So, it does run on PHP5, just doesn't use PHP5 only features for the sake of backwards compatibility. For PHP5 one can use KohanaPHP, as mentioned by another comment :)
edit: what arien said. plus, again, if it doesn't fit your needs, you can jump from CI to something else pretty readily.
Best of luck