My experience with the Sunday Assembly in my area was that it was really nice, but a pale comparison of the church life I had experienced.
I wish I knew what the missing element was, but I'm afraid a big part of it is the opt-in nature. It's like the difference between a good family and a good group of friends. The former is held together not just based on a desire to be together, but (usually) also on a sense of obligation/duty or even deeply held belief.
