We are going to switch from an homemade framework to Symfony or Yii. The capabilities of these two framework are, from our point of view, similar (ie they both have what we need).
Our situation : php 5.2.9, medium-size website (10.000 uniques / day) but shared hosting.
Please no answer like "this one is better", ideally someone that would have use the two frameworks recently.
I'm extremely late to the game here, but it's kind of relevant because Symfony2 is actually stable and in use. I struggled to get Symfony2 working for a week and a half on a work dev box running CentOS 5.6 VPS (with a relatively locked down configuration, but I can sudo).
Then I quit because the "Agile Development with Yii" and "Yii Cookbook" I had ordered a few days earlier had arrived, and I thought to myself "nothing could be worse than this headache". Basically, there was all kinds of issues with git and Symfony's console depending on it (I think). I was just trying to play.
Then I started playing with Yii. I designed my db very carefully, checked out the extensions, including the bada$$ bootstrap extensions, and basically my entire app was done in about 48 hours, and it included a 10 table db, tons of data, and fairly complex user permissions, etc.
Piece of cake. I don't really see the advantage of Symfony2 other than the fact that it's going to be the backbone of Drupal 8, if that's the kind of thing you're into. :)
Oh, and not only did Yii make it easy to build the app, it made it easy to make me look like a ROCKSTAR. Total jQueryUI integration (in addition to Bootstrap - which is responsive by default, so I don't even have to worry about that crap). All data is delivered through portlets and widgets that are super easy to use.
I couldn't recommend Yii highly enough. It's the bomb.