I've used it in the past (and also contributed some patches), it has some roughness around the edges, but it works far better than Neo4J.

It handles far bigger dataset than Neo4J (even if performances are not great) and it is easier to use and maintain.

Uses little memory, no JVM. Also interesting for golang shops since it's codebase is relatively small and clean.

Neo4J is open source.

> The Neo4j Community Edition is licensed under the free GNU General Public License (GPL) v3 while the Neo4j Enterprise Edition is dual licensed under Neo4j commercial license as well as under the free Affero General Public License (AGPL) v3.

http://neo4j.com/open-source-project/

How is your environment "polluted" when running something on the JVM?

It is hard to explain - having JVM on the server is like having empty cans of beer laying around in the living room.