Any experience with cstore_fdw? It looks like there is a (finally) an open source columnar and compression extension for Postgres.

I wonder if anyone had any experience with it yet? 

I've worked with Greenplum for years, but I've been looking desperately for the open source alternative.
you get an impression of the performance looking at the comparison with a real column store: 
https://www.monetdb.org/content/citusdb-postgresql-column-store-vs-monetdb-tpc-h-shootout