In case you drag and drop our controls on a regular Sharepoint aspx page or user control, you can take advantage of its design-time as in any other regular Visual Studio web site/web application project. If you create your web part dynamically and add the respective RadControl to it, you will not be able to utilize its design-time capabilities.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.