I have developed more than 7 web projects using Symfony framework. Mobshare.in, Amvizone.com, Campaignmanger.in , Movieonline.in etc are some of the websites. But for the past 5 months my developement is in Codeigniter. It is very very lighter than Symfony and has almost all the features of Symfony. Both are MVC framework. But symfony has 4 layer while the latter has only two. Symfony uses modules (extra layer) for Controllers. But Codeigniter eleminates that layer. It is similer to Ruby on Rails.
Symfony uses naming convention to map controller and view. For example controller index need indexSuccess.php file as view or indexError.php as error. But in Codeigniter, we have to specify the view manually from the controller
Symfony has command line tool to develop all the model classes. One thing you have to do is specify the scheme.yml file. But in Codeigniter, you have to make model classes from the scratch. However a lot of database operation libraries are available with it to make the coding life easier. Since we have to load each libraries manually, Codeigniter gaurantee the speed of execution. (Autoloading option is there, but in most of the case we can eliminate unwanted libraries from autoloading – just like execute on the fly)
Codeigniter has a simple folder hierachy that everybody can understand and very less learning curve than Symfony.
Go through this link if you wanna know about all the features of codeigniter : http://codeigniter.com/user_guide/toc.html
I found another interesting comparison of these two in : http://www.devtrench.com/codeigniter-vs-symfony/