Check out http://techblog.netflix.com/2012/07/benchmarking-high-performance-io-with.html
Netflix used Cassandra with SSDs and were able to drop their memcache layer. Mind you they were not using it purely as an in memory KV store.
I'm thinking about using Cassandra as an in-memory key/value store instead of memcached for a new project (just to get rid of a dependency if possible). I was thinking about setting the replication factor to 1, enabling off-heap row-cache and setting gc_grace_period to zero for the CF that will be used for the key/value store.
Has anyone tried this? Any comments?