I've started experimenting with CakePHP yesterday. This was actually the first time I've used any framework and even though I don't completely grasp how it works yet, I was already pretty impressed by what I could build up in just half an hour (using Sitepoint's CakePHP tutorial).
The idea with CakePHP is as follows:
Cake is a rapid development framework for PHP which uses commonly known design patterns like ActiveRecord, Association Data Mapping, Front Controller and MVC. Our primary goal is to provide a structured framework that enables PHP users at all levels to rapidly develop robust web applications, without any loss to flexibility.Personally the main advantages I take out of this is that using such a framework forces you to use a certain approach to programming, which usually means following a good practice one. The MVC (Model View Controller; seperation of data, output and business logic) approach brings more structure into your applications and with the help of all the built in functions provided by CakePHP is very powerful, yet also quite flexible!
I recommend any PHP developer with some spare time on their hands to take a look at this tutorial and the CakePHP website. The website contains a pretty elaborate manual with detailed installation and configuration instructions (don't worry, takes about 5-10 minutes). Getting started with CakePHP with just the manual was a bit difficult though, so that's why I recommend the Sitepoint tutorial. This tutorial will take you around 30 minutes to complete and really gives great insight in what CakePHP (or a development framework for that matter) is all about.
Update #1: acutally this post on Giga Promoters Blog has some very interesting things to tell about CakePHP before you get started.
Update #2: another great tutorial by IBM. Actually part of a series. (requires registration with IBM, free).