Has anyone successfully integrated Apache Mahout with Neo4j as a datastore? If so, how much works was involved, and what was the performance like?
I haven't heard from anyone that's tried it. There is no neo4j-specific connector code in the project.
To the extent you can use neo4j with Hadoop, you can use it with Mahout; Mahout doesn't have a lot to do with particular data stores.
That's the good news, I suppose; there's not much about Mahout that's particularly tied to a data store, so yes with a little work I'm sure you can get something working.
I don't think a graph database is the most natural choice to store the sorts of stuff Mahout crunches, which are just big vectors.