The upshot is that Ruby and Rails are better tools for rapid development with a small team, but that they don't compare to the JVM for encapsulating things so that multiple teams can change things at the same time. There is also a performance gain, but that's a secondary concern.
Twitter seems to have taken a very smart route here - build their product quickly with "Agile" tools, then change as needed to support a larger user base.
I'm particularly geeked (pun intended) about their choice of JVM language. They went more cutting-edge with Scala as their language of choice for most applications.
