RadEditor for SharePoint 2010 which is part of SAK is an official and stable product.
Basically, the links inserted in the content area of RadEditor remain untouched but when the content is submitted it is validated by the SharePoint filters which can update them. My suggestion is to enable the MakeUrlsAbsolute filter of RadEditor and test your scenario similarly to my test shown in the following video: http://screencast.com/t/E1vJgMERp
the Telerik team
Learn how the Telerik controls can be integrated in SharePoint 2007/2010 from this resource
. To watch them in action, explore our online SharePoint 2010