My company&#x27;s main app (~50k LOC acording to rake stats) has been upgraded continuously from 1.2 up to 4.2.  The most painful upgrade was 2.3 to 3.0, followed closely by 3.0 to 3.1 (which was incidentally painful because of a performance regression affecting ruby 1.8, so we had to simultaneously do the 1.9 upgrade at the same time which was the most painful ruby upgrade of recent memory).  Since then the upgrades have been easier.<p>From what I&#x27;ve seen 5.0 should be on par with the 4.x upgrades, which is to say, not bad.  YMMV of course.