Edit: At the Oxid Commons Conference 2014 the developers of the popular web shop „Oxid“ announced that they will migrate their System to Symfony 2, too. Here’s a blogpost by their community manager Marco Steinhäuser, explaining that they have chosen Symfony 2 over Yii despite Yii’s recent surge in popularity.
A beautiful description of why Symfony 2 was chosen despite the fact that a lot of time and energy had already been invested into the Yii Framework can be found in this article.