SQL Server Analysis Services - who uses them? We all know Sql Server Analysis Services (SSAS) delivers online analytical processing (OLAP) and data mining functionality for business intelligence applications.

Who has used this service in their projects? I need some guidance on what this really involves?
Analysis Services is a very large and widely used part of the MSSQL stack, your question is entirely too broad to give an accurate answer. I will tell you that a typical SSAS project involves more than just OLAP; you have to shape your data first. A typical scenario starts with one or more sources of data which have disparate structures. An ETL process (often times performed using a tool like Integration Services or SSIS)  is used to gather that data, perform operations such as cleansing and ultimately drops that data into a data warehouse/mart which is a central database typically designed using a star or snowflake schema. (this is a world unto itself with lots of debate but think of a more flattened data design). From this warehouse you use SSAS to design cubes which are built from the data in said warehouse and once they are complete you browse those cubes with everything from Excel up to an including highly specialized reporting environments.

Again, your question lacks specificity so it is hard to explain things beyond that without writing a small book.