For everyone, they're coming and fast - I've found a nice little crash course for smarty from the good folks at php.net:

http://smarty.php.net/crashcourse.php

The full documentation can be found here: http://smarty.php.net/manual/en/

"Why use it?

One of Smartys primary design goals is to facilitate the separation of application code from presentation. Typically, the application code contains the business logic of your application, written and maintained in PHP code. This code is maintained by programmers. The presentation is the way your content is presented to the end user, which is written and maintained in template files. The templates are maintained by template designers.

At its most basic function, the application code collects content, assigns it to the template engine and displays it. The content might be something like the headline, tagline, author and body of a newspaper article. The application code has no concern how this content will be presented in the template. The template designer is responsible for the presentation. They edit the template files, adding markup and bringing it to completion. This typically involves things like HTML tags, cascading style sheets and other tools provided by the template engine.

This paradigm serves several purposes:

*) Designers can't break application code. They can mess with the templates all they want, but the code stays intact. The code will be tighter, more secure and easier to maintain.

*) Errors in the templates are confined to the Smartys error handling routines, making them as simple and intuitive as possible for the designer.

*) With presentation on its own layer, designers can modify or completely redesign it from scratch, all without intervention from the programmer.

*) Programmers aren't messing with templates. They can go about maintaining the application code, changing the way content is acquired, making new business rules, etc. without disturbing the presentation layer.

*) Templates are a close representation of what the final output will be, which is an intuitive approach. Designers don't care how the content got to the template. If you have extraneous data in the template such as an SQL statement, this opens the risk of breaking application code by accidental deletion or alteration by the designer.

*) You are not opening your server to the execution of arbitrary PHP code. Smarty has many security features built in so designers won't breach security, whether intentional or accidental. They can only do what they are confined to in the templates."

There's a full library of plug-ins available here:

http://smarty.incutio.com/?page=SmartyPlugins

To add to this; a lot of providers use Smarty with their programs, they are definately nice to fool with and can give you a lot to play with as well (as far as scaleability goes); various *Nuke's utilize Smarty for their implimentations as well.