I did it in reverse, from Hong Kong to Shanghai in June. I will admit, it was a long trip. The scenery was really interesting, however. It was my first glimpse of China and I was surprised at how non-dingy it was along the railway. Since it's viewed as an international train, you leave China and go through customs in SHanghai and then go through HK customs, etc when you get to Hong Kong - you don't get off the train at any points in between.
The food that came around didn't look all that appealing. I had brought a full complement of snacks onboard - those ramen cup noodles, for example, since they have hot water on all of the trains. They came around with fresh fruit a few times, beer, water, drinks, etc. and all were very reasonably priced.
I took the hard sleeper class - as a solo female traveller, I wasn't comfortable with the soft sleeper option since those doors can be locked from the inside.
Carry your bags on - I'm assuming that you won't have too much? I had a large backpack and a smaller daypack, plus my ever-important food package. I saw other Westerners getting on with rolling suitcases. You can store them under the lower beds, on an empty one or above the door where there's a little shelf.
As for the trains themselves, they were extremely clean and well maintained. The best train I had ever taken in China. Bring eyeshades and earplugs if you desire to sleep at anytime other than 10-7ish. They open all the curtains around 5 or 5:30, and the horrid canned music comes on sometime after 7. Bedding and pillows are provided, and (after a careful inspection) were clean - comfy white blankets.
The hard sleeper sections have six beds per berth and you pay according to which one you want. The highest one is cheapest, the lowest the most expensive. The problem with a lower berth is that people use it as a couch during the day. I opted for the middle.
The soft sleeper cabins have four beds per berth, lockable doors. It's also more expensive. I travelled soft sleeper on different trains, and based on anecdotal evidence, I decided that the increase in price wasn't worth it.
I think you can also get a deluxe soft sleeper with only two beds per berth. The price increases quite a bit with each of these options.
Hard sleeper = squat toilet, soft sleeper classes = western or squat. The bathrooms were surprisingly clean.
You can get tickets at China Travel Service...
If you have any questions, PM me. I had to do a ridiculous amount of research on the train question to appease my parents, including pros and cons of each class to justify my decision.