Symfony 2.0 has been in use on large websites for over 2 years now, and was a major framework release in the PHP community. It was one of the first frameworks to truly take advantage of the language features available in PHP 5.3, and it has had a huge impact in shaping the next generation of PHP frameworks such as the newly announced Zend Framework 2.0. The Symfony standalone Components have also been used to replace large chunks of code in popular PHP projects such as Drupal8 and PhpBB4.
The release of Symfony 2.1 is great news, particularly with regards to the Form Component which I have been experimenting with recently. I personally have jumped ship from using the Zend Framework (ZF), to using the Symfony 2.0+ stack on newer projects. I have started implementing the Symfony Components within older ZF 1.0 web apps. And on smaller projects I will be experimenting with Silex, the Symfony-based "micro-framework".
With the community slowly adopting PHP 5.4 this is the time to ensure you are keeping pace with the newer frameworks and that you have moved into the post-5.3 era.
Symfony Standalone Components: