Go to the p&p Smart Client Guidance Community Site, download the current release and start playing with the samples out of the Smart Client Contrib package. They rewrote the original BankTeller sample. By comparing the original BankTeller sample with the one out of the SCSFContrib you'll see the advantages of a WorkItemController over a typed WorkItem.
The samples just show you what you can do with CAB, but to start developing your own app you need a deeper understanding of its mechanisms.
Therefore, I would propose to read the introduction of Rich Newman - it is the best I ever read. To me, it was most important to get a comprehension of the WorkItem and how it is used by SCSF (WorkItemController). Don't reduce it to a representation of an "use case", it is much more.
To build an application with CAB rely on Workspaces (CAB), the WorkItemController (SCSF), SmartParts (CAB), Services (CAB), Events(CAB), Commands(CAB) and Actions (SCSF). Don't use UIExtensionSites and the WorkItem.State.