I have completed migration of our large application from 3.4 to 4.1.1a. The migration process was long and while not difficult, it required me to look at every line of code.
As a result, I was able to do significant refactoring and make sure our layouts complied with the best practices listed in the performance guide. I did not implement MVC.
Has anyone else had this experience? I'm not talking about simple, one page web apps, I'm talking about large business applications.