Scene7 is not a CMS but rather a hosted cross media platform for managing, enhancing and delivering dynamic, rich content to any channel - web, print, email, and mobile.
Typically, clients would use S7 alongside any CMS. It's particular well-tailored for companies who have a lot of visual assets to manage (images, videos...) and need dynamic image serving capabilities as well as image viewing features such as zoom, spin etc.
Where it becomes really powerful is by using its template functionalities which allows for parametrization of assets and thus serves as the foundation of personalized dynamic content while leveraging a CDN for delivery.
There are also inbuilt transcoding capabilities - a way to upload, edit, manage and publish high-quality video experiences to multiple screens including desktops, mobile devices and tablets without IT support.