Some of the features are:

• Built on top of PHP 5, for me that’s a plus since I’m only working with PHP 5 at the moment.
• MVC support makes application building faster and easier.
• CRUD generator will reduce code repetitions and useless boring time spent on building this tasks.
• Lots of support, when you start learning something new, all the support available you can get is a plus.
• And a lot of other features, such as: Ajax support, smart URL’s, cache management, multilingualism and l18N support, scaffolding, among others. You can check the features list at Symfony Project website.

The only thing I think it’s a bit hard, is the learning curve. What I mean is, you really have to “waste” some time to get to know how the application works, but I think in the end it will compensate.