We've worked on it for over a decade, it's used in production by thousands of community users, hundreds of customers and 75+ Global 2000 companies (see http://neo4j.com/customers). For many of those Neo4j is used in business critical use cases, i.e. they require Neo4j to be up and running every minute of every day or it'll show up in their next earnings call. If you've shopped online or in a US retail store this week for example, it's very likely that you've used Neo4j. There's rich support for pretty much any programming language and framework out there, an ecosystem of consulting partners whose sole business it is to do Neo4j implementations, 10+ books written specifically about Neo4j, rich online training, formal enterprise support backed by a global commercial organization, an active community. What's missing?
I'm not trying to be facetious -- I'm genuinely curious as to what you feel is missing to consider it mature.