I've been forced to fly MU quite a lot during the course of the past year because they have a quasi-monopoly on one of my most common routes. While I don't have a single truly nightmarish experience to share (even if I did, you could take it with a grain of salt; even good airlines have off days on occasion), my general feeling is that those guys are reliably dismal. Basically, out of 30 flights, not a single one has been even close to on time, and MU doesn't seem to care in the slightest about this. I know that endemic delays are more of a China-ATC problem than an MU specific problem, but the fact that they continue to schedule a 6p departure from NNG to SHA and the inbound flight never makes it to NNG before 6p is seemingly something that they could fix (e.g. call it an 8p flight from the start).
On a more positive note, I do have a 100% success record of scoring good seats: (reclining) exit rows, bulkhead aisles, or upgrades (business on their 321s is surprisingly comfortable). However, I would be willing to put up with the worst seat on the plane if they took some meaningful steps to ensure overall comfort.