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The benefits (Pros) of switching to the latest versions are:
- Support for all modern browsers
- Modern look and feel - many new modern skins are available: Bootstrap, Material, Metro and other improved skins
- Modern lightweight rendering - less HTML, usage of modern CSS3 and font icons instead of images
- Easier customization of the skins and ability to create new themes through the Theme Builder tool
- Responsive rendering and mobile support for many of the controls
- New controls to complete the portfolio of more than 100 Web UI components for ASP.NET Webforms
- Better stability and less bugs
The cons of upgrading to the latest version are:
- The classic render mode and skins based on it are obsolete and put in maintenance since Q3 2015 (see documentation)
- Some classic skins have been removed, but are still available as download resouces.
- Breaking changes - see Known Issues and Breaking changes sticky thread