I won't enthuse about the virtues of web-services and especially REST - many people have written great articles and presentations before me :-)
One problem irked me: delivering different content on the same URL using a framework.
I'm building a system which stores data about a company.
You visit company/20 and you get a page with data about company 20.
You POST to company/20 and the system saves the new data.
This can make your controller a little fugly:
<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
$this->_saveCompany();
}
else {
$this->_viewCompany();
}