Symphony
 

I created the "First Project" tutorial, and I liked how quickly modules could be created, and the built in functions seemed to be pretty nifty.

I wanted to see what others have run into with this structure, and if it's worth getting familiar with. The only concern I have with it at the moment is how difficult it would be to add fields and do other upgrades to an existing project.

What is "Symphony"?

Opps =( Symfony.

http://www.symfony-project.org/

I can't say I've ever used Symfony. I've tried CakePHP, which I didn't take to, but now I'm with Zend Framework, I love it. Admittedly it has a few issues, but because of its ease-of-use, adding in layers of your own that interface with Zend Framework is so damn simple.

Does Symfony allow you to use bits and bobs of it, or does it throw you into a framework which you must follow if you want to use their helpers and functions?

I'm going to try out the Zend framework sometime soon. I'm glad to hear you've had a good experience with it.

From what I've experienced so far with Symfony I can add in custom code, but it's all contained within their structure and naming conventions. I don't know how flexible their structure is at the moment.

Does the Zend framework have any AJAX support/functionality?

I can't wait to sink my teeth into both of these and get into the more advanced features. :)

yes, it has ajax support, check this:
Zend Framework: Documentation
Zend Framework: Documentation