Woohoo! Redis has quickly become my favorite transient data store. I'm using a combination of Postgres, Redis and Cassandra and feel like I have the natural data mapping for every scenario. No more pushing square pegs into round holes.
Redis is a swiss army knife, Cassandra is a Ka-Bar.
* Redis is much easier to understand, since Redis is a key-value store with rich datatypes (such as lists, sets and hashes). Cassandra is a database paradigm shift build to solve Google scale problems.
* Redis perfomance is similar to memcached - and memcached's perfomance is pretty amazing.
I would only pick something like Cassandra if I had to store a lot of data on a large number of computers. If I wanted to do something fast and I know that my data won't grow to terabytes then Redis is a very good choice.
In a nutshell I use Postgres for the relatively static data and Cassandra for the very high volume transactional data. I use Redis for ad-hoc data structures and some caching. The natural fit means I'm not jamming de-normalized data into a relational database for performance reasons. Hope this helps.