We're just off a 30 day cruise on the Ryndam with flexible dining.
Our experience was mixed. As an introvert I found it exhausting meeting new fellow diners 3 times a day. So many conversations never got much beyond, 'How many cruises have you been on? How many children/grandchildren do you have? What was your work? etc. We met some delightful people with whom we planned a repeat dinner. With them we enjoyed real conversations. We met a few who left us exhausted trying to dream up open ended questions to keep up some form of social interaction.
But a real complaint was being seated with a couple of other couples who were half way through their meals. One time they were being shown desserts while we were eating our first course.
After several times either racing to catch up with the earlier diners or skippin a course, we learned to state that we'd only sit with others IF they were just seated. There are still kinks for HAL to work out.