Recs.
Updated
SpecsUpdate
Pros
Pro Easy debugging with a built-in debug toolbar
Symfony comes with a built-in toolbar that helps developers debug their applications during the development phase.
The toolbar is also extendable and new components, called panels can be added if needed to help with the debugging process.
Pro Great templating engine
Uses Twig, which is a simple and easy to learn templating language that can also be used as a standalone engine, outside the framework.
Cons
Con Promotes bad development practices
Such as annotations via comments.
Con Doctrine ORM
Symfony Standard Edition, which is the most widely used distribution, comes integrated with Doctrine, the most resource hogging ORM library.
Con You need a lot of files to display a single page
For a simple hello world page you need about 5 files.
Recommendations
Comments
Out of Date Pros + Cons
Con Bad documentation
The documentation is poorly written, so learning Symfony can be difficult, especially for beginners.
Con Low performance
Probably the slowest PHP framework out there.
Con Somewhat high overhead
Symfony can add overhead in terms of CPU and RAM usage as demonstrated by many benchmark tests.