In a recent entry to his blog Andy Jeffries gives an overview of his site's transition over from one framework to another - away from Symfony and over to Rails.
I decided as I was learning/using Symfony at my contract at the time that it would be a good experiment to write it in Symfony. [...] The site did fairly well when I posted but I still posted in frequently.
After redefining the site's purpose a bit, he reconsidered the language (and framework) choice and decided to do a rewrite in Rails. He gives a comparison of the process on things like lines of code, time to build, performance differences, deployment and automated testing.