I cannot comment on long-haul business class, but I certainly don't agree that Hainan Airlines is the best mainland Chinese airline for flights within China. In fact, I would put it among the worst.
Air China, China Eastern, and others offer an extremely pleasant flight experience between major mainland Chinese cities. Planes are comfortable with good entertainment, they always serve a substantial meal, and they even have some nice lounges (especially Air China's PVG lounge). I am an American and I also tend to find the service lovely and the English level is quite good among their crew.
Hainan was actually my worst Chinese airline experience, and I've even flown some of the mainland Chinese low-cost carriers. The flight attendants are not trained in even the most basic safety precautions. There was a huge line to use the bathroom midflight and when extreme turbulence hit (to the point where the captain asked FAs to sit), the FA just let the passengers continue to stand in the aisle. Eventually, she had them crouch in the aisle instead of returning to their seats. She had absolutely no idea what she was doing and it was scary to imagine how she would handle an emergency. Also, they somehow spilled a large amount of urine in the galley floor. Overall, a terrible experience and I try to avoid Hainan domestically, though it might not be an issue for long!