A great article on the NoSQL movement, focusing on Graph Databases and the Neo4J implementation appeared on InfoQ.
Graph databases store a Node (aka vertex), that has properties (aka attributes), that is then linked to another node. The relationship (aka edge) between the nodes also has properties and has a direction and a type.
The article demonstrates that syntactically these databases look easy to manipulate. In Ruby (and presumably Grails) they map directly to classes and with the use of metaclasses and operator overloading, to allow the domain objects to be quickly in a neatly expressive way. For Groovy/Grails/Griffon, there is info on the Neo4j wiki
Because OO models are graphs, the need for ORM layer is removed. They map directly to the classes we are trying to store. Additionally the pain of upgrading the data in a class is reduced since adding a new column is less painful. More benefits can be found on the Neo4J website.
Neo4J is free for open source projects and there is an unsupported free single instance Basic edition for startups, but is paid for everyone else. There are 3 commercial editions, basic, advanced and enterprise priced at $588, $5,988 and $23,998 per annum (billed as monthly subscription). The free Basic is a good move, small companies who may elect a free alternative such as MySQL, Postgres or even the Oracle Express Edition. For the Basic & Advanced editions, you could argue that the development time saved would more than cover the fees charged.
The basic edition lacks monitoring which in my opinion should be included, especially if you are only going to get 2 support incidents per year with that package. However the advanced edition has that feature plus monitoring, a management console and allows hot backup and failover to a 2nd running node. It only differs itself from the high end one due to lack of high availability features and longer support response times. If you were using it for a production enterprise system, you’d probably want the 24/7 support and I think this pricing is comparable to a support subscription for any of the larger RDBMS vendors and with no upfront capex fees to note, though keeping in mind that at the enterprise level you’d be paying for multiple running nodes costs may run up quickly —- there is always negotiation though in this space.