Sitefinity CMS is a stable, powerful, and easy to use CMS. It just works! For you, the developer, this may be one of the reasons why you might have never considered upgrading. The most recent releases pack multiple enhancements that might tip the scales.
To switch or not to switch? If you are still reluctant to make the move to the ASP.NET MVC framework, this blog will help you by outlining how your organization can benefit from MVC.
The Progress Sitefinity teams recommends to all Sitefinity developers to use MVC whenever possible, especially for new projects. The MVC approach is superior in terms of performance and productivity for developers compared to WebForms.