Jim Menard's Alternate Data Storage Technologies is an excellent outline which delves into what type of data store to use in an app. The when to's, when not to's, and real-world cases brought up here are especially relevant to those who might want to use RDF as an alternative to a relational db (or not).

Object Databases

• Store graph of objects
• Handle inheritance and circular references
• Generally tied to a particular OO language, though can have adaptors for other languages
• Efficiently traverse relationships
• Don't need to worry about whether an object is in memory or not
• Handling schema changes can be difficult (see discussion in In-Memory Databases below)
• Let's Use an Object Oriented Database from the WikiWikiWeb.

Source