<p>What's the best/easiest to integrate templating system for PHP, and what are the benefits of using one?</p>

<p>I currently don't use one at all, and am thinking that it might help to seperate content from presentation a little more. </p>

Smarty
I've found it to be fast, easy to use, and easy to install (even in a shared-hosting environment).  It also doesn't require that you use validating XHTML which is handy sometimes (although I think the template engines that
do
require valid XHTML are probably faster.)
It's really nice to have your content in one place and code somewhere else.
Plus you're not limited to just HTML.  I've used Smarty to generate
XML
and even
SQL
.