Smarty is not an application development framework. Smarty is not an MVC. Smarty is not an alternative to Zend Framework, CodeIgniter, CakePHP, or any of the other application development frameworks for PHP.

Smarty is a template engine, and works as the (V)iew component of your application. Smarty can easily be coupled to any of the engines listed above as the view component. No different than any other software, Smarty has a learning curve. Smarty does not guarantee good application design or proper separation of presentation, this still needs to be addressed by a competent developer and web designer.