If you are new to MVC frameworks, you can get started very quickly with Codeigniter or Kohona. Otherwise, it's really worth investing some time learning Symfony or cakephp.

Another way to go would be to learn a ORM (doctrine) and a templating engine (tinybutstrong or smarty). I have never done any big php project without the help of frameworks, but some of my work friends find this approach (templating engine + ORM) more easier and better. I can't agree with them, though.

Frameworks are not all about rapid development. If used properly, they structure your application and make it easier for maintain.