Security explorer makes it worth the money
Site Administrator for SharePoint runs within the Quest Management Console for SharePoint, and is one of a suite of five products Quest provides for SharePoint. It includes four tools: a site browser, a report manager (the reports are not very useful), a policy manager (very few people use this) and a recently added Security Explorer.
The site browser is the core of the product. It allows you to browse through all of your SharePoint sites and the objects they contain, via a treeview control on the left. Right-clicking on any object in the treeview provides a submenu of various tasks. For instance, right-clicking on a document library gives you several options, one of which is "Customize". Choosing "Customize" brings the document library's settings page into the right pane of the management console. Being able to access and act upon all of your SharePoint sites and objects from one console like this is a real timesaver.
The "Security Explorer", which allows you to control all users and permissions for all of your SharePoint sites plus perform powerful filtered searches for users, groups and/or permission type, is a big plus. It's inclusion makes Site Administrator worth the price tag.