It doesn't "auto save" but the new workflow IS improved.
Before, when you ended and saved your workout it dumped you out of the Workout app completely. If you were just switching exercises (like treadmill to rowing) you had to reopen the workout app... which was annoying. Now, it's much quicker when switching activities.
Also, it happened to me quite a lot that I would end my workout but NOT save it. What this meant is that the next day I would go to start a workout and be presented with the save screen for the previous day... which meant I had to hit Save.... be dumped out of the App.... restart the app and THEN finally be able to start my activity. It's definitely better now.