I have upgraded a very large Rails 3.2 app to 4.2 recently. It was about 2 weeks solid of work for my team. It was a project I inherited and when I got it the test suite took an hour to run.<p>The biggest problem which I had was dealing with the dependencies. This project used just under 300 gems, many of them unmaintained and a few of them I had to fork to get to work with rails 3.<p>I am very fast making things in Rails, for the most part I find it just works. When projects get to a certain size, very roughly around 50k loc, they become very slow to work on. Slow test suites and dependency hell become problems.