I flew MU aplenty, both domestic, international and TPAC. In Y and J.
Hard product is comparing advantageously to others in many cases. They have a variety. Their B777-300 J product is pretty good. Y is cramped just like any other airline now it seems.
Service in the air is also decent. NH/JL/KE/OZ it is not but it certainly beats CA, which has quite a bit of an attitude in too many cases.
Food in J is good too. Entertainment system is good.
So, what's there not to like?
Just one thing: their ground game. That is where the proverbial s*** hits the fan.
If you don't have IRROPS and delays, you are good. But if your flight is canceled or delayed, then you maybe in for quite an experience. I got through a number of them. In most of them, there is generally no signs, no explanations besides "please stand at some place" that should be evident but isn't. When you arrive at such a place, lo and behold no one there to service you perhaps. Or perhaps later. You don't know.
During these times, be prepared to rely on the help of your unfortunate comrades that happen to speak Chinese (like me) because there may be no official announcement, no helpful, Japanese-style helper trying to make sure you stand at the right place. This is especially the case for domestic flights or when you are transferring to domestic flights. Situation becomes fluid. Confusion reigns supreme. With some luck, you won't end up like me last time - left all on our own at midnight at PVG T1, zero MU agent to take care of a whole 777 plane that needed to go back to the gate.
And be ready to take matters into your own hands. Book your hotel on your own (the hotels they book for you may be horrible anyway) and be ready to call their number to discuss the next steps. If someone can speak Chinese and can help you, even more the better: you get the line faster and they are much more polite in Chinese than in English (which can be quite curt).
At least, this has been my experience. Again, 15 years of flying with them.
Now, would I recommend them? Yes... and no. It depends on you. If you are a seasoned traveler, or even somebody with decent experience, you should. You will save money, have a great product (in J at least) and most probably there won't be a diversion or a snafu like I had. They aren't fumbling the ball all the time either. But if you just cannot deal with snafus at all and expect to be taken care of in case of a problem, then I would suggest flying Japanese, Korean, perhaps also US airlines. Unfortunately, Chinese airlines and MU are a bit more rock and roll there. You need to be more of a self-starter and have more patience in such cases.