For my use case, I require extremely high write throughput but don't care much about durability. I also require indexing for fast queries.
I've been using Redis with RediSearch, but I've always desired real SQL queries.
In general, as RAM becomes cheaper and acceptable server response time shrinks, I think we will trend toward in-memory databases. MongoDB for example has an in-memory storage engine you can purchase. I do like MongoDB and Redis, but when you are dealing with relational data SQL is really the best fit IMHO.