SharePoint Designer (SPD) is a great Rapid Application Development (RAD) tool for building workflow (Document and other) for the Microsoft Office SharePoint Server 2007 product. It is a tool that both developers and non-developers can use, albeit at a different level of capability. That being said, you can get a lot done without writing code with this tool. If you are a Visual Studio Developer you will find it a great tool to quickly start building a Workflow, and since it uses the same Workflow Engine as Visual Studio (i.e., Workflow Foundation), you can open the workflow's build in SharePoint Designer and modify/enhance them in Visual Studio 2005 and vice-versa.
If you want to see it in action, check out my recorded Webcast in which I use the product to build a custom document workflow here.
~ Robert Shelton, Jr.