MetaCPAN has been using Elasticsearch for a long time... a really long time, we last 'upgraded' to version 0.20.2! Our current setup relies on a single box and this very old version, and it works great, that said...
We have spend the last couple of years (this is a voluneer based project), doing major work in reoganising how we index and have update all our code, such that we now have a 3 node cluster (and long term we will have 2 datacenters with a cluster in each) running Elasticsearch version 2.4.0 this will give us reliability and access to a vast number of extra features, from autocomplete to aggrgations.
We think this will benefit not just https://metacpan.org/ but the Perl community as a whole, who can access these new features through our api https://fastapi.metacpan.org/