Aspose.Words is a very good tool to have if your work includes creating custom documents. It supports a wide array of programming languages like .NET, Java, Android, etc. The API is easy to use and is simple to understand even for beginners, this is one of the main reasons Aspose really shines and has made creating custom documents an effortless task. Earlier I was using Microsoft Interop, but that is not the recommended way to create word documents in ASP.Net web application.

General Usage:
Aspose.Words also makes it easy to create, modify and replace all document elements such as sections, headers, footers, paragraphs, lists, etc. It even lets you use formulas, all drawing objects such as shapes, text boxes, images,etc and even add water marks, and protect documents and convert documents from one format to another. It can also be used to view and print documents.

Formatting Features:
All document elements can be customised extensively and easily. It also allows you to copy fragments from one document to another, modify it's properties and also add custom HTML code. Aspose.Words also allows you to use all these formatting features even for existing word documents like .DOC(97-2003), .DOCX and OpenOffice documents.

Mail Merge Feature:
One of the main use cases of Aspose.Words is that it allows us to use standard Microsoft Word mail merge fields in your document. It allows you to create, modify and replace these fields easily. Aspose.Words also helps in replacing Mail Merge fields with complex tables. It can do all of this by using data from any data source. It also has a very simple syntax for creating an MS Word Mail Merge Field.

Document Object Model:
Aspose.Words uses the Document Object Model. This makes it easier to create complex documents in a short time. It can also be used to create a summarised report by combining fragments from multiple documents.

Export Features:
It helps you to create, modify and export your created document in number of different file formats like DOC, OOXML, RTF, HTML, OpenDocument, PDF, XPS, EPUB and other formats, you can export hundreds of documents in a short time, all in different formats quite easily. It has also reduced the lines of code for converting a document to another format to just about one to two lines of code.

It allows you to do all of the above mentioned features and more without having to install Microsoft Office or Microsoft Word on the system which helps in completely removing the dependency on Microsoft Word. It also supports all versions of .NET Framework.