We switched over to these shortly after they were available. The primary solution to the technical problem facing Rails apps on Heroku (made broadly known by the excellent Rap Genius blog posts) was to switch from the stock 'thin' web server to 'unicorn' or similar web server. 2X dynos make this solution even more effective, by providing two times the RAM which allows you to run more unicorn workers per process (e.g. from 2 to 4 for us) and have allowed us to reduce our Heroku bill by a couple hundred dollars a month. On top of that, the double CPU share means that workers will generally return the same results faster. It's sort of a no-brainer for Rails apps.

The other big win for us on Heroku recently was that Adept Scale came out of beta, which is a Heroku add-on which will monitor your traffic levels, response times, and related error rates and automatically scale up your dynos as needed. I'm not really sure why this isn't a core feature of the Heroku platform. We pay $36/mo. for Adept Scale, but they save us probably $200/mo, at least.