I've been on one Mega Ship (Voyager of the Seas) and I was pleasantly surprised how well designed this ship was to handle crowds. I had thought there would be lots of lines, but there really was no problems at all. In fact, it didn't seem as crowded as the Conquest.
I'll be sailing on the new Celebrity Solstice in February which is also a Mega Ship at 133,000 GT, but it has fewer passengers than most of the big ships. It'll be interesting to see how they all compare.