This thread will likely get a million "us too" style posts but Redis is core infrastructure at Shopify. It's been so solid that we recently waived our defensive requirements that the app remains working if Redis goes down. This allowed us to port our inventory reservation system (a huge point of potential lock contention) completely to the new server sided LUA scripts. We have seen a full order of magnitude speed increase from this. A reservation for a complicated order is now measured in µs instead of ms.
I'll put "us too" here, as we too now assume Redis to be up like our relational store(s) -- Percona MySQL. ~16GB at ~500 ops/s averaged over the last 3 months. We've been running Redis for core features for more than 21 months, and as a store, it has been the most stable and easiest to reason with for what performance we can expect and actually receive. Compared with "non-SQL" setups we deploy, if we were to start from scratch, we'd look to replace ActiveMQ, Solr, and a number of our jobs that we jam through MySQL.
