I've flown MU from LAX to PEK vis PVG and had really mixed results. The flight out was half empty and I had a lot of room to sprawl out and relax. The cabin was spartan and I don't know that it was super clean. The flights back were full and I had screaming children behind me (not MU's fault). It was certainly not a glamorous experience, but all my flights were on time and it was by far the least expensive option. I would probably fly them again for the right price.

Worst roundtrip flight in my life
 

I bought a round-trip ticket from China Eastern and they lost my luggage both times!! I arrived in Sydney, Australia with no fresh clothes or underwear and had to live that way in a hotel for 2 days before they finally found it. No apology, no money refunded.
Coming back home they lost my luggage again! I'm still waiting for them to find my 2 bags, their phone numbers either are out of service or just keep ringing with no option to leave a message.
Besides having the worst phone service and losing my luggage constantly, the plane is dirty, completely uncomfortable, puts pork in your vegetarian meals, the flight crew have terrible attitudes and were very rude, they only show Chinese commercials or Chinese movies with no subtitles, and yes the plane does have a certain putrid smell. I couldn't listen to the radio because it only had Chinese artists, the films were all in Chinese with no dubbing, and there was absolutely nothing I could do on a 30 hour plane ride from NY to Sydney.
I even remember when they tried to hand me a burger and I told them I'd rather have some peanuts because I don't eat meat, they were so rude and told me to 'Just Try It.' I had to beg them for 2 bags of peanuts instead, which they handed me with such disdain.
I paid $1,100 of my hard-earned money (which was very hard to save) and got treated like a stowaway in a ship's cabin. It was absolutely dreadful and I urge anyone considering using their airline to Please Re-think it.
I'm never using them ever again!!

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.

I flew MU earlier this month in biz class from SIN to PVG as part of a DL award itinerary. Given the dismal to mixed reviews on-line for MU, I was pleasantly surprised. A newer Airbus A330 (although with Shanghai Airlines, rather than China Eastern, livery). Comfortable angled lie-flat seat (2-2-2), cabin only half full. Flight attendants were pleasant and attentive, not the sour and dour attendants you often find on Chinese airlines. The food and wine were fine. Departure and arrival on time, and after I cleared Chinese immigration, my checked bag was on the belt. Only negative was very limited IFE choices, but that's why I bring a tablet. Absolutely nothing to complain about.
http://www.catskill4sale.com/images/museat.jpg

http://www.catskill4sale.com/images/mumeal.jpg

Flew with them this past month to Shanghai, Hong Kong and Bangkok in coach. Crew was better than expected, but most of the flights were delayed and the food was awful. I don't think I'll fly with them again unless there is great fare, just because none of my flights seemed to leave on time (the bad food I can live with because I would skip it).

For sha-hkg I dare say that most people who have to do the drill on a frequent basis would choose ka y over mu j when given the choice (usually about the same price). For sha-sin theccalculus is a bit tougher (mu j v. Sq y) because that's a pretty long flight which ups the comparative value of the nice seats, but even then I always choose sq (I don't take red eyes). You see, when you find yourself in singapore with the intention of flying to shanghai on mu and your airplane doesn't show up you are pretty much hosed because they simply don't care.

I flew a RTW in biz earlier this month on three alliances - Star, OW, and Skyteam. I flew the last two segments on MU (HKG-SHA & PVG-SFO):
* The HKG-SHA flight was short. I did not remember everything except that service and seats were all very good.
* The PVG-SFO flight was excellent. It is of course better than UA and BA, but it is even better than CX. I do like that CX AVOD more though. MU screen is farther away (~1.5m from my eyes) and there seemed to have fewer selections. The flight attendants were all very attentive. :)