The Windows Azure blog has a long and good interview with Jonathan Ellis about Cassandra and a lot of other relevant topics. Take a break to read Thought Leaders in the Cloud: Talking with Jonathan Ellis, Co-Founder of Riptano. When asked about Cassandra vs. RDBMS, this following is interesting:
I think relational databases are going to stay important. They solve some important problems, and there’s a very rich ecosystem of tools around them, which keeps time to market low. I see Cassandra as particularly appealing to companies that started on something like SQL Server and then reached the point where favorable price/performance to buy larger machines isn’t there anymore. The pain they’re feeling from the pressure to scale is greater than the pain of learning a new technology like Cassandra.
So people using relational databases are looking to move to Cassandra, mostly because of the scaling aspect, also sometimes for the reliability aspect. Cassandra deals very well with multiple data centers, in terms of preparing for one or more of them failing and clients having to access a different one.