I have used Open Xml SDK for document generation in SharePoint, PPTX generation for custom presentations (pulling from XSLX for the data) and for "building" composite documents from multiple document segments.
The format and the SDK are great. No worries on the server in ASP.NET or SharePoint scenarios, and great speed. I have not found too many scenarios where the SDK or "brute force" xml cannot accomplish a goal. One instance is password protection and DRM for documents, but these are more corner cases. I would agree with aaronaught that this is not an exclusive solution, but SharePoint, VSTO and others are tools in the tool belt for document generation solutions.
