It's a tedious time sink. While I don't deny that having great powers in the upper floors can be fun (or frustrating if you were unlucky) it's an immense timesink that gets boring pretty quickly.
If you just want to progress with 3 chars you spend more than 10h there in this week alone, doing the same in the same environment with virtually no rewards - not even vanity - at the end. Just as much the gameplay is advanced over island expeditions, at the same rate the reward structure is a huge step backwards.