Intersystem's Cache can be defined as Object Oriented Database.
The way I think of it is that it is regualr RDBMS database with Object Oriented scripting engine. Oracle has stored procs,Cache has Object script.
We are migrating to Intersystems Cache to take advantage of their BI & reporting tools. I still haven't run across a situation that cannot be solved with stroed procedure of MySQL or Orcale yet. I prefer to write everything in SQL procedures to avoid any future migration headaches.
People with strong object oriented background might prefer using ObjectScript.
Just in case you haven't seen it, here is link to their documentation: http://docs.intersystems.com/cache20101/csp/docbook/DocBook.UI.Page.cls?KEY=GIC_INTRO#GIC_C96028