We went to Night of Joy on our first trip. It was our first night, so I can't tell you how the parks were during the day, but after that day, the parks were empty. I don't think many people from the resorts go (yes, there are quite a few, but compared to the crowd as a whole). I think most of the people that go are teenagers from different youth groups around Florida. With that said, we were very disappointed in the crowds. We were mid-twenties then, and it was just my husband and myself. We didn't fit in well with the loud young teenagers. Michael W. Smith was there, though, and he's my favorite. So, Michael W. Smithin Walt Disney World couln't be any better. If you do choice to go, be prepared for a lot of teenagers that aren't well supervised.
After that night, I didn't notice crowds at all. I would guess that a lot of youth groups make day trip to Walt Disney World for the event, but don't stay for any length of time.