I don't know about you, but to me this seems like a big blow to the reputation of Cassandra. I know a bunch of people (including myself) whose primary motivation for investigating Cassandra as a large-scale data store was "Twitter is using it, therefore it's got to be fine for what we want".

In my experience this was not the case -- after quite a lot of prototyping, we moved away from Cassandra. It scales brilliantly, but its query time has a pretty solid floor that I was unable to shift to speeds approaching what I needed: it never went up, but nor did it go down.

Of course, I am a total noob at distributed systems -- perhaps it's simple to get it to go faster, but Twitter's move away suggests not. (Facebook's use-case is quite different, and in any case they still use an awful lot of MySQL too) The other systems they cite as currently using Cassandra in production are nowhere near the scale of the primary tweet store.