For the most part, I loved the cooperative dungeon experience. I also especially loved the puzzles and battles in the dragonspiral tower. My constructive feedback echoes almost everyone else in this thread.
1. Because this dungeon is so difficult and so involved, you sort of needed to do it with a lot of real-time coordination. So not having a way to assemble a team of your friends without other random people joining was really annoying. There should be a way to make a group other than random groups or guild mates.
2. If the guilds can do it with 3 people, the other groups should be able to leave with 3 people.
3. Having the Lucarionite and the light stone be RNG dependent was really tough, since this quest took so much time and effort. If only one was RNG dependent, that would have been better, but having neither be dependent would be much preferred. Especially because if your friends get the good rewards, they don't have any incentive to do the quest with you again, so it gets harder to get the rewards as your friends stop wanting to help.
Overall, I liked the collaborative quest and would enjoy more things like this in the future.