It's a good product, and it was even fairly easy to do a major version upgrade / cluster relocation. At least as easy as such a thing can be. :-)
Your queries tend to break down into ones where you're hitting a small number of shards (such as when we serve data for the analytics page), or else ones where you tend to aggregate up into temporary, non-distributed tables for further analysis (such as you'd do for infrequent business reporting).
All told, it's been one of the easier tools for us to operationalize, largely thanks to the fact that it's "just PostgreSQL." The one thing I wish we had was better documentation of what kind of consistency guarantees to expect, although for an append-only store like our current use case, that's less of a concern. And to be fair, Ozgun and the other guys at Citus have always been really happy to answer any questions we have.
With this code now going open source, it should be pretty easy to look into these sorts of internals.
It looks good, will have to wait and see what actually makes it into 9.6 and how well it supports all the possible queries. This is something that many commercial databases have solved so Postgres is pretty behind in vertical scaling.