So I've done a few missions now and had some time to form some opinions, good and bad. First, the good.
The Diadem is fun. Just flat out fun. There's something so cathartic about a huge mob of people murdering the heck out of everything that crosses their path. The brachiosaurus in particular is an epic, epic fight. And it's a nice, easy (relatively) way to bang out some esoterics. ...When it works.
But that's only when it works.
- The damage threshold to get credit on higher ranked mobs -- specifically V's and stars -- is way too high. Way too high. If your group is down even one person, you will not get credit on V's and stars, and even IV's can be iffy.
- Needing to attune to aether currents every time you go in. Specifically this is bad because it often ends up being an "every man for himself" situation. Those that are lucky enough to avoid aggroing mobs -- or who take advantage of their party members trying to be helpful and grabbing threat -- run off to grab the currents on their own and leave the others in the lurch.
- Leeches. The ones who go off to gather and leave the rest of their party in the lurch, but still get to roll on what chests the group is able to earn, which is few because, again, the damage threshold is punishingly high.
- The two hour lockout. When the above three points combine and a group falls apart, it feels like a punishment for something that isn't even your fault.
Now, I understand the high damage threshold is there specifically because of my second and third points; it's trying to prevent people from taking advantage of others' work. But when all those things are happening at once, it makes the Diadem a pretty miserable experience, not worth the time, effort, or aggravation.
ETA: If you're going to make it a requirement for a full party of 8 people all playing their best to get credit on higher ranked mobs, at least let us replenish our ranks if/when someone drops early in the mission.