From a Ruby perspective, but this may still be useful: Redis is very nice for data that fits in memory (disk persistence is for recovery, not for realtime access) and support for counters, sets, etc. is cool.

That said, I really like MongoDB for many reasons: interactive shell, great Ruby support (and Scala and Clojure, etc.), very easy to set up and use, and some replication support (not as good as Cassandra, but I will never need that kind of scalability).

I think that the Python support for MongoDB is very good.